Andreas Weber1, Mohammad Ibrahim Khalil1,2*, Martine Schraml1 and Reinhold Gutser1
1Department of Plant Sciences, Institute of Plant Nutrition, Technische Universität München, Germany
2UCD School of Biology & Environmental Science and Climate-Resilient Agrit-Environmental Systems (CRAES)-UCD Earth Institute, University College Dublin, Ireland
Received: 21 August 2018
Accepted: 03 October 2018
Version of Record Online: 15 October 2018
Weber A, Khalil MI, Schraml M, Gutser R (2018) Soil-Atmosphere Exchange of NH3 and NOx in Differently Managed Vegetation Types of Southern Germany. J Soil Plant Biol 2018(1): 01-15.
Correspondence should be addressed to
Mohammad Ibrahim Khalil, Ireland
Copyright © 2018 Khalil MI et al. This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and work is properly cited.
Ammonia (NH3) and Nitrogen Oxides (NOx = NO + NO2) emissions from soils and vegetation, and their subsequent deposition are key factors in global Nitrogen (N) cycling and have important functions in atmospheric and ecosystem degradation processes. To better understand their contribution, NH3 and NOx gases were simultaneously measured from differently managed vegetation types using a dynamic-chamber method. Biomass and N yields were higher from unfertilized clover-grass than fertilized oilseed radish. Summer cuts of clover-grass resulted in 137% higher biomass and 2.7-3.7% N concentrations than autumn cuts. Mulching reduced the re-growth and biomass production in clover-grass by 16% compared to cutting. The relative loss of NH3 through mulching was higher from the clover-grass (2.18%) than in the oilseed radish (0.08%). The total NH3 release over the four cuts of the clover-grass was 0.58% of the N removed. The influence of biomass-N, either mulched or cut, on the total NOx emission was temporary, resulted in net deposition (0.02-0.15% of the added/removed biomass-N). The ecosystems acted as sources for NH3, with the rate being weakly related to the added biomass-N, air temperature and humidity (R2 = 0.58, p<0.07), and sinks for NOx, with the rate influenced significantly by sunshine hours, precipitation and amount of biomass-N added (R2 = 0.87, p<0.001). We conclude that cutting clover-grass multiple times could be a good option to reduce the emissions of reactive N species and increase fodder yields with moderate N. Additionally, clover-grass could be superior for soil conservation measures over oilseed radish. Results imply further studies on the annual exchanges of gaseous N between the ecosystems and the atmosphere through long-term measurements.
Keywords: Mulching/Cutting; Forage; Temperate Climate; NOx Deposition; NH3 Loss
Ammonia (NH3) and Nitrogen Oxides (NOx = NO + NO2) play a vital role in global Nitrogen (N) cycling through their emission from soils and vegetation, and their subsequent deposition onto soil-plant systems [1,2]. Both compounds also have important functions for various atmospheric and ecosystem degradation processes. Their deposition onto both terrestrial and aquatic systems results in acidification, eutrophication and a decrease in plant biodiversity . The NOx is also responsible for direct or indirect contributions to global warming and ozone depletion. There are several factors governing their production, and that the contribution of agriculture to their overall release and the potential for smart management practices to reduce the emissions are complex and multi-faceted.
Compared to global average, the highest NH3 emission rates are found in many of the regions in Europe and its sources and influencing factors have been well discussed . The average NH3 emission in Western Europe is 12 kg N ha-1 yr-1 , and emissions from decomposing crops used as mulch are ranging from 17 to 39% of the total N present in the mulch. Gauger et al., reported an N deposition of 24 kg ha-1 for Dürnast and 27 kg ha-1 for Viehhausen , whereas Werner et al., estimated 2.3 kg NO-N ha-1 yr-1 emissions from grassland systems . Vegetation/grassland can act as sinks of soil-emitted NH3 and NOx depending on the soil mineral N concentration . However, agriculture remains the main source of NH3 volatilized to the atmosphere and that the management of soil-plant systems is equally important to that of animal rearing [1,9].
In addition to the organic matter turnover in the soils, the plants themselves emit NH3 to the atmosphere via leaf senescence and N remobilization [10-12]. This process is accentuated by the presence of NH4+ and/or the application of NH4+-based fertilizers, particularly those derived from urea . Both serve as additional major sources of NH3 and can lead to excessive N absorption by the roots followed by high N concentrations in the foliage . The precise level of NH3 emission from various systems depends on substrate availability and its chemical composition, soil management and other environmental factors. Moreover, NH3 volatilization is stimulated by the difference between compensation point over canopy cover and atmospheric mole fraction, and the external N supply [14,15].
Mulching is of particular concern for NH3 production and that is thought to impact its emission on a par with that arising from live plants from their metabolic processes . Ammonia emission from mulched material probably derives from metabolic enzymatic breakdown, and its potential magnitude depends largely on the concentration of NH4+ in the plant tissue . Nitrogen oxides, by contrast, have short atmospheric lifetimes and their real-time concentrations vary depending on their proximity to their sources. For instance, soil represents a net source of reactive oxidized nitrogen as NOx [16,17], and natural sources represent only 30% of the total emissions of these compounds . The biogenic emission of NO (or NOx in general) is a surface-related process, with the compounds being formed as intermediates or by-products of nitrification and denitrification processes that are regulated by heterogeneous microbes under both aerobic and anaerobic conditions [2,19]. The amount of substrate/sources and their rates of N turnover, either organic or inorganic, control NOx production potential and there are opportunities to limit its emissions from agriculture by investing in management practices . This production potential is also influenced by physical and chemical properties of the soil and NOx partitioning in plant biomass and is highly variable particularly due to natural or anthropogenic disturbances .
Agriculture has a potentially important role to play in reducing its contribution to N deposition through the adoption of appropriate management options [4,22]. For example, grassland farming without grazing can impact significantly on N cycling through the process of cutting the grass at predetermined intervals to feed to the animals as either forage or hay. Promisingly, however, strategic management of legumes in an arable-ley rotation to enhance fixation of atmospheric N and recycle the stocked residues in grassland could increase productivity, enrich soil N pool and reduce reactive N species  or vice-versa [24,25], and may effective for soil conservation. Finding solutions to the above issues are hindered by the limited amount of data and poor understanding of their functional relations with soil and environmental factors underlying them. To address these deficits, we conducted experiments (i) to evaluate the biomass and N yield benefits from clover-grass mixture (mulching and cut) and oilseed radish receiving inorganic N fertilizer; (ii) to quantify the losses of NH3 and NOx from these ecosystems; and (iii) to elucidate those variables related to climatic conditions and to the biochemical composition of the vegetation that affect NH3 and NOx emissions.
Two separate field experiments were conducted during the vegetation periods in two consecutive years at the Viehhausen and Dürnast research stations of the Institute of Plant Nutrition, Technische Universität München (TUM) in Freising, Bavaria in Germany. Both soils are silt loam from loess (Cambisol) and some of their physical and chemical properties are presented in table 1. The Viehhausen station is located 5 km south of the TUM, where a long-term experiment on clover (Trifolium repens L.)-grass (ryegrass mainly: Lolium perenne L.) management was established by the Bavarian State Research Center for Agriculture. This site received no fertilizer for the two years preceding this study (Table 2). Prior to that, the crops in Viehhausen were potato (Solanum tuberosum L.) and winter wheat (Triticum aestivum L.) receiving manure at 200 kg N ha-1 and inorganic N as Calcium Ammonium Nitrate (CAN) at 60 kg N ha-1, respectively. The Dürnast station is located 6 km west of the TUM, where an experiment with oilseed radish (Raphanus sativus L.) was set up following the cultivation of winter barley (Hordeum vulgare L.). At this site, CAN was applied at 60 kg N ha-1 and the straw of the previous crop of winter wheat, receiving cattle slurry at 240 kg N ha-1, was incorporated (Table 2). Prior to the winter wheat, winter barley and maize (Zea mays L.) were cultivated, which received mineral N as CAN (60 kg N ha-1) and cattle slurry (200 kg N ha-1), respectively.
Experiment 1: NH3 and NOx fluxes from a clover-grass mixture: The experiment at the Viehhausen station was established to investigate the N use efficiency of a clover-grass mixture, consisting of 70% white clover, 25% ryegrass and 5% indigenous legumes, in rotations. The clover-grass was harvested four times during the growing season (May 10, June 29, August 16 and October 17), keeping ~5 cm of biomass above the ground. The treatments consisted of the harvested biomass (i) laid on the surface for subsequent mulching i.e., mulched and (ii) removed immediately after cutting i.e., cut. The treatments were arranged in a Randomized Complete Block Design (RCBD) and the data derived from both periods of harvesting and utilizations were analysed by Two-way Analysis of Variance (ANOVA). Representative dry weights of the mulched and cut clover-grass used as fodder were recorded from a whole plot (10 m x 5 m in size) using pseudo-triplicate sub-samples. Pseudo-triplicate plant N and Carbon (C) concentrations were also determined using a C-N analyzer (Vario MAX CNS, Germany). Simultaneous measurements of NH3 and NOx were performed over a two-week period by placing three chambers (system details follow) on the soil surface immediately after mulching and cutting for each measurement plot (representing the number of replicates) and period of harvesting.
Experiment 2: NH3 and NOx fluxes from oilseed radish: At the Dürnast station, a catch crop (as cover crop cum mulching) of oilseed radish were grown separately following winter-barley cultivation. The treatments consisted of N fertilization applied to the oilseed radish at a rate of 40, 80 and 120 kg N ha-1 with CAN two weeks after sowing and arranged in a RCBD. Plots of 10 m x 5 m were selected for each ecosystem/treatment. The plants were cut three months after sowing (October 20) from the whole plot and mulched. The corresponding biomass and N yields were recorded. Both NH3 and NOx fluxes from the ecosystem were measured using a single chamber for each plot over a period of three weeks starting immediately after mulching.
Both NH3 and NOx were measured concurrently using a specially developed dynamic-chamber (area 0.125 m2, diameter 0.40 m and height 0.40 m) method. The system had six chambers (Figure 1) with a facility to collect ambient air 3 m above soil surface and to ventilate at a continuous airflow, measured daily using a Thermal Mass Flow-Meter (TSI Model 4045, TSI Inc., USA), of ~40 L/min. Each chamber was covered with acrylic glass fitted with a stainless-steel ring and was inserted the sharpened bottom to 3 cm soil depth. An angled inlet and additional ventilator were used to flow the ambient air horizontally 5 cm above the soil surface to mix with the gas emitting from the treatment plots within the chamber. Then it was flowed upward to the outlet having gas sampling point. All tubes (Fluorethylene-propylene Teflon) were insulated and heated along with the chambers to 50°C with an electrical heating cable.
The continuous gas sampling flow (4 L/min) was verified by a Mass-Flow Controller (AFC 50D, France) before the sample was assayed by a Two-Channel Chemiluminescence NO-Analyser (CLD 700AL, Switzerland) at a flow of 0.7 L/min. In one channel, a stainless-steel thermal converter was used to convert NH3 + NOx to NO under a vacuum at 600°C (NOx + amines), and in another, molybdenum one to reduce NO2 to NO at 375°C. Ammonia concentrations were calculated as the difference between channels 1 (NOx + amines) and 2 (NOx). The NO-analyser was calibrated daily using NO, and a Portable Calibrator (VE3M, Germany) was used to check the efficiencies, ranging from 97 to 99%.
Emissions were continuously measured during the experimental period, representing flushing time as well, with consecutive sampling for 15 minutes and the last point was used for data analysis. The NH3 and NOx fluxes were calculated as the difference between the concentrations of the respective gases in the collected ambient air and in the air sample at the chamber outlets. The chambers were moved to a new undisturbed area of the same treatment plot up to three times a day to minimize any possible greenhouse effects (e.g., humidity, temperature) on gas exchange inside the chambers. The system was controlled using a PC via the software NEMO Lite v3.70 (Schmidt Technology, Neufahrn/Munich, Germany), which recorded data for each of NH3 and NOx and the airflow.
The ANOVA for each experiment separately and statistical analyses were performed using the computer package JMP v4.0.2 (SAS Inc.). For dry matter and N yields, the probability (p) values were determined by the F statistic and specific differences between pairs (Two-way ANOVA: periods of harvesting and utilizations for clover-grass and one-way ANOVA for oilseed radish) of means were measured using Tukey’s Honest Significant Different tests. Total NH3 and NOx emissions/depositions were calculated by integrating the daily fluxes, and standard errors. Simple and multiple linear regression analyses were performed for NH3-N and NOx-N emissions/depositions (with or without a mathematical transformation) with selected meteorological variables, biomass N and soil temperature.
Forage Dry Matter (DM) yields of the clover-grass mixture varied significantly among the four periods (p<0.0001) and between the two management strategies/utilizations (i.e., mulching versus cutting; p<0.001), but not between the periods and utilizations (Table 3). For the first two of the four cuts (from May to June), the management system influenced the biomass production significantly. The DM yield was significantly higher for the June cut (5.20 t DM ha-1) than for either of the mulched or cut plots. The forage yields for the fourth cut in October were less than half of the three earlier cuts, regardless of the management system. Generally, DM yields of clover-grass for all harvests were higher in the cut plots than in the mulched ones. There was a significant (p<0.001) difference in biomass N concentrations for the clover-grass over time, but not for the management systems and their interactions, with increased N concentrations later in the season (August and October cuts). Nitrogen yields differed significantly between periods (p<0.0001), utilizations (p<0.01) and their interactions (p<0.01). The highest value was observed for the mulched plot during third cut (142 kg N ha-1), despite N yields being generally lower in the mulched plots.
Values with the same letter do not vary significantly to each other; NS = Not Significant.
The DM yields of oilseed radish were dependent on the amount of fertilizer applied with the values for 80 or 120 kg N ha-1 applied as CAN (6.86 or 7.17 t DM ha-1) being significantly higher (p<0.01) than those for 40 kg N ha-1 (5.26 t DM ha-1; Table 4). The N concentrations in the oilseed radish were likewise increased from 1.6 to 2.2% with increasing fertilization rates, showing no significant difference, and were lower than that as observed for the clover-grass. The addition of 80 or 120 kg N ha-1 to the oilseed radish also significantly (p<0.001) increased N yields over the lowest rate. After six months, the total biomass production and N yields from the oilseed radish were two-fold lower than from the clover-grass either mulched or cut. As determined from biomass production, the apparent uptake of soil N in excess of that from the CAN applied to the oilseed radish ranged from 38 to 64 kg N ha-1, with the highest value derived by the application of 80 kg N ha-1.
Values with the same letter do not vary significantly to each other; NS = Not Significant.
Ammonia volatilization from the clover-grass plots was consistently influenced by both the measurement period (first through third cuts) and the management systems (Figure 2). The peaks for NH3 fluxes from the mulched plots (maxima of 198 to 864 g N ha-1 d-1) were generally higher than those recorded from the cut plots (57 to 273 g N ha-1 d-1 from first through third cut). Fluxes for NH3 differed significantly for most of the measurement days, with the largest peaks appeared within three to nine days after mulching or cutting. An exception was observed after the fourth cut, where the highest peaks occurred later (e.g., the maximum peak of 566 g N ha-1 d-1 occurred on day 13). The total NH3 volatilized over the four harvests showed similar trends (Table 5), with the mulched plots emitting about three-fold more NH3 (9065 g N ha-1) than the cut plots did (2740 g N ha-1).
The annual average ambient concentrations of NH3 and NOx was 46.0 ± 5.7 and 9.8 ± 1.2 ppb, respectively (mean ± standard error); NS = Not significant.
In contrast, NOx exchange was dominated by deposition (Figure 3), with the peaks for emissions/depositions being largely inconsistent in their timing between the mulched and cut plots. The exception was after the first cut, in which both management systems produced overall NOx emissions (mulched: 14.4 g N ha-1 d-1; cut: 12.2 g N ha-1 d-1). From the second cut onwards, the fluxes decreased substantially, showing only small emission peaks or large depositions. Over all, the net amount of deposition was 206 and 235 g NOx-N ha-1 from the mulched and cut clover-grass plots, respectively (Table 5).
Following some short peaks that occurred around day 12, the plots mulched with oilseed radish produced their maximal peaks for NH3 after day 15 (231 g N ha-1 d-1) on day 18 for those plots initially received the highest amount of inorganic N (Figure 4). The application of inorganic N at lower rates of 40 and 80 kg ha-1 did not show much residual influence on NH3 volatilization, showing the maximum peaks of only 10.3-10.5 g N ha-1 d-1. Over the course of the entire two-week measurement period in the mulched oilseed radish, the highest amount of inorganic N (120 kg N ha-1) released significantly (p<0.01) greater total NH3 (255 g N ha-1) than other two N treatments (42-47 g N ha-1) (Table 6).
Values with the same letter do not vary significantly to each other.
Figure 4: Measured and predicted total NH3 and NOx fluxes (g N ha-1) over a two-week period with the corresponding regression equations for the dependent variables influencing their emissions/depositions for a pooling of clover-grass, legume mixture and oilseed radish that was either mulched or cut (a; NH3-N = -142.62 + 0.34 NR + 5.20 AT +1.44 RH, R2 = 0.58, p<0.07, n = 12, and c; NOx-N = -242.10 + 16.46 SH + 1.49 P + 0.33 NR, R2 = 0.87, p<0.001, n = 12) and mulched only (b; 1/NH3-N = -0.0163 + 0.0016 C/N - 0.0002 P, R2 = 0.80, p<0.05, n = 12, and d; NOx-N = -316.63 + 18.06 SH + 0.77 NR + 1.63 P, R2 = 0.95, p<0.01, n = 12).
(NR = biomass N added through mulching in kg ha-1; AT = mean Air Temperature in oC; RH = mean Relative Humidity in %; C/N = Carbon and Nitrogen Ratio; P = total Precipitation in mm; SH = mean Sunshine Hours; n = Number of Samples)
Deposition of NOx was generally observed into the oilseed radish plots throughout the measurement periods, with the net rate not differing between the management strategies (Figure 4). The only exception was a peak in NOx emission at day 6 for those plots mulched with oilseed radish that received the lowest amount of inorganic and biomass N. The total NOx deposition over a two-week period from plots mulched with oilseed radish varied significantly (p<0.01), ranging from 103 to 135 g N ha-1 (Table 6).
We performed correlation analyses relating the daily NH3 and NOx fluxes with the daily changes in soil temperature and selected meteorological variables for all crops, either individually or in combination. All variables, either individually or in combination, showed a very poor fit to the daily NH3 fluxes. By contrast, daily NOx fluxes showed significant correlations with some variables, although the predictive levels were generally low (Table 7). For instance, fluxes of NOx were related to relative humidity and soil temperature for mulched or cut clover-grass (R2 = 0.13-0.33, p<0.05). For the mulched oilseed radish, the best predictive levels obtained with precipitation and air temperature (R2 = 0.38, p<0.001).
Correlation analyses were also performed between total NH3 or NOx fluxes over the two-week measurement periods with mean values of air temperature, relative humidity, sunshine hours, total precipitation, and the amount of biomass N applied through mulching. Although not correlated at the 5% level, the square root of the total NH3 flux was positively related to the combination of biomass N rate, mean air temperature and relative humidity with a predictive level of 58% though increased sample size showed a significant relationship (Figure 4a). When the mulched input variables were taken into consideration, the total 1/NH3 flux showed a strong relationship with C/N ratio of the mulched materials and total precipitation (R2 = 0.80, p<0.05; Figure 4b). However, the best fit to the NH3 flux, either normal or transformed, by any single variable was with %N (R2 = 0.54-0.59, p<0.05, n = 8).
The total NOx flux for all three crops in combination, by contrast, was best explained by the combination of mean sunshine hours, total precipitation and biomass N rate, showing a predictive level of 87% (Figure 4c). Considering only the mulching effects of the legume species, the predictive level for the total NOx flux for this combination of variables improved to 95% (Figure 4d). The total NOx flux was generally not significantly related to any single meteorological variable, although the trends (R2 = 0.35-0.69, p<0.02-0.13, n = 8) were stronger than those derived from the biochemical characteristics of the biomass (data not shown).
Values for biomass (as either forage or mulch materials) and N yields for clover-grass (70% white clover) from either the mulched or cut treatments differed markedly to those for the oilseed radish. The yields were higher for clover-grass, attributing to a higher N input received through Biological Nitrogen Fixation (BNF), compared to the fertilized oilseed radish. Mulching somewhat decreased the yield benefits for the clover-grass, probably due to the resulting physical barrier, and shading-induced lower temperature compared to cut practices and thereby limited the re-growth of the clover-grass. The oilseed radish apparently exploited up to 64 kg of soil N in excess of inorganic N applied, a value that was several-fold lower than the N (soil + atmospheric) yielded by the clover-grass.
A well-established process of BNF largely compensates for any insufficient application of inorganic N in the production of clover-grass. As typical forage legumes could fix atmospheric N2 in amounts ranging from 196 to 240 kg ha-1 . Moreover, the mineral N derived from decomposition of organic materials, especially following cut practices, might contribute to future N availability, leading to facilitate enhanced N uptake and vegetation growth. This conjecture is supported by other workers for example Herrmann et al., reported a compensation process through BNF when inorganic N was applied at a low rate . The plant components of clover-grass contained higher N than did the oilseed radish and thus yielded more N. Indeed, the biomass and N yields from each cutting of the clover-grass observed here were within the upper limits of recorded ranges .
In addition to the advantages of mulching practices for soil conservation measures, legume species can also contribute to enriching the soil pool with nutrients to supply to future crops in rotations. By contrast, application of inorganic N is imperative to boost biomass production of oilseed radish. Oilseed radish can be used mainly as a cover crop and/or mulching material but contains low N and generally shows poor nutrient release over time due to a large C/N ratio .
Mulching forage materials influenced NH3 emissions significantly, with large peaks appeared three days after its placement. The levels of volatilized NH3 from this mulched clover-grass system were three times greater than those found under cut conditions. The precise timeframe, when the maximum peak appeared, varied slightly over the seasons for clover-grass and its delayed appearance for oilseed radish due to its higher C/N ratio. These observations could be ascribed both to the natural variation in the meteorological variables and N immobilization phenomena that reduce NH3 emissions [25,29,30].
Residual N following the application of inorganic and organic N fertilizers to the previously grown crops might also have contributed to the large NH3 losses. This contrasts with oilseed radish, which is an exhaustive crop. As such, the amount of fertilized N lost as NH3 was noticeably the lowest for the mulched oilseed radish (0.03-0.16% or 0.08% on average). The percent of N lost as NH3 from the mulched clover-grass (2.7% of the N concentration during crop residue decomposition) was either within or exceeded the upper ranges reported by Mannheim et al., . Some researchers, however, observed values that were several-fold higher than ours either in laboratory-based studies using selected parts of various legume species or in long-term field studies [5,25].
In line with other research , our findings demonstrate that the mulching of the clover-grass having high N concentrations could significantly increase the atmospheric build-up of NH3. By contrast, Mannheim et al., stated that NH3 emissions from crop residues could be reduced substantially through ploughing and mulching . Ploughing could indeed be an effective management strategy . However, the loss of other N species may be high during denitrification , whereas mulching-induced N2O loss could be less than 1% [24,25]. Taking the value of 1.5% added-N lost as NH3 from CAN into account [29,30], mulching of the oilseed radish seemed to reduce the NH3 volatilization for prior N application rates of up to 80 kg N ha-1 compared to the clover-grass. Riedo et al., also reported reduced emissions 12 days after cut/fertilization events, a result that was even more pronounced after the second cut event compared to the first one .
Outright removal of the clover-grass after each of the four cuttings to use as either forage or hay reduced the overall NH3-N loss three times more than when it was mulched. The re-growth of plants required after cutting enhances ammonium availability in the soil, leading to an increase in apoplastic ammonium (i.e., leaf tissue N) and thereby NH3 emissions [10,12,35]. However, mulching with legume vegetation might override these processes. As such, NH3-N emissions from the cut clover-grass were higher than those observed from the mulched oilseed radish. In the former case, this can be attributed primarily to the contribution of the biologically active soil N pool. The N pool generally derives from legumes through microbial symbiosis as well as decomposition of roots and nodules, and/or newly growing leaves [10,12], with a subsequent influence by the air temperature appearing to have a small influence . It was not possible to clearly distinguish the microbes responsible for BNF either directly during the re-growing periods or interactively through NH3 exchanges during soil C and N mineralization. Under field cutting conditions, the NH3 emissions between the first and fourth cut were 0.58% of the biomass N removed through cutting, including accounting for the uptake that occurred in some instances. Herrmann et al., however, reported that NH3 deposition could exceed its emission under field cutting conditions from a clover-grass mixture, even when applying inorganic N fertilizer and assuming a deposition of <1% of the N removed by cutting from the system . The NH3 levels emitted from the mulched oilseed radish with its higher C/N ratio were very low, which may be attributed to N immobilization and is comparable to the findings of Herrmann et al., . By assuming an estimated loss of NH3 from the soil N pool of ca. 201 g over a two-week period, a net deposition of NH3 could be observed from mulched oilseed radish receiving low N rates from either inorganic or organic N sources . This indicates that NH3 emission from catch crop species after mulching might not exceed the atmospheric concentration of NH3 or might be close to the canopy compensation points. These could demonstrate either low net emissions or overall deposition if any losses are held to have occurred from the soil N pool.
The NH3 emissions overriding zero/canopy compensation points were regulated mostly by the quality or biochemical composition of the substrates mulched (as delineated by %N, C/N ratio or biomass N added, either alone or in combination with selected meteorological variables) and relating to the C and N mineralization processes [15,28]. Similarly, Glasener and Palm thought that the high lignin and polyphenol concentrations in N-rich plant materials might be counteractive to NH3 emission . Moreover, several researchers have indicated the importance of the metabolic enzymatic breakdown of cells/mulched materials for NH3 emission. Its overall contribution depends largely on the concentration of NH4+ in the plant tissue and often exceeding that of the stomata of the plants [10,22]. The C/N ratio of the plant components (considering mulching effects only) in combination with precipitation influenced NH3 volatilization significantly, and %N as a single variable also contributed to this effect in agreement with the findings of Larsson et al., . Increased NH3 loss from vegetation having high N concentration, mainly as ammonium in leaves (and in litters), has been reported .
We also found that NH3 emission was inversely, and weakly, related to the water content of the plant components. This result contrasts with those of Mannheim et al., who stated that the plant components of a mixed crop having high water content emitted more NH3 than did plant parts with high dry matter and N concentration . These could be explained by the differences in moisture content between the mulched materials used in this study being relatively small compared to those used in Mannheim et al., . Our results also indicate that NH3 emission was influenced by the combination of the amount of added biomass N, air temperature and relative humidity. Thus, interactive effects of the water content of the plant components and meteorological variables could also exist. Similarly, Mastrorilli et al., revealed that, following cover crop (Vicia faba) decomposition, NH3 emission stopped during rainy days and recommenced with the increased amount of solar radiation, soil and air temperatures . Besides, NH3 emission is high and frequent (occurring about 50% of the time) during warm, dry summer periods. By contrast, deposition is dominant (80% of the time) in wet, cool autumn periods due to small canopy compensation points caused by the low temperatures and generally wet surfaces [15,38]. Soil evaporation, plant transpiration and ambient atmospheric humidity all contribute to humid conditions. Therefore, any dynamic changes of canopy liquid water storage could regulate the internal cycling of NH3, leading to enhanced deposition or degassing of the highly water-soluble ammonia [4,10]. Our results further reveal that the biochemical composition (C/N ratio, %N) or the amount of biomass N added in association with selected meteorological variables could also significantly influence NH3 volatilization.
When comparing the different mulched materials, use of oilseed radish seemed the best option to reduce NH3 emissions. Still, oilseed radish degrades slowly, and the presumed N immobilization would result in a poor supply of nutrients to the crops. Instead, the full NH3 emission scenario needs to consider both applied inorganic N fertilizers as well as the entire farming system approach being used. Thus, multiple cuttings of the clover-grass, using either as forage and hay or as an N-rich mulching material to improve the soil N pool for the next crop in rotation, might be a better alternative to oilseed radish for ecological farming.
Deposition of NOx was dominant for the clover-grass strategy; the only exceptions were after the first harvest (for both mulching and cutting) and after the second harvest when the biomass was cut and removed. The deposition of NOx was slightly greater for the cutting than for mulching strategy over the two-week measurement periods, indicating only a small influence for added biomass N. The total exchange of NOx over the course of 2-3 weeks was noticeably higher in the oilseed radish, with deposition increased with decreasing amounts of added N, than in the clover-grass. Although the added N influenced NOx emission [17,20], both soil and environmental conditions might lead to overall deposition over the short measurement periods used here. Some researchers reported the uptake of NOx by vegetation canopies from the point of view of N deposition , with its consumption through canopy reduction possibly being up to 50% of soil emission . By contrast, Werner et al., estimated a NO emission potential of grasslands of about 2.3 kg N ha-1 y-1 . Our results suggest that NOx deposition in the grassland systems might be the dominant process, at least during summer and autumn, and particularly so for either the mulched or cut re-growing grass covers without additional inorganic N input. In line with the observations of other workers , the increasing concentrations of ambient NOx we observed from May to October might be one of the important factors relating to NO2 uptake by the soil and/or low soil NO emission and canopy resistance. Finally, Ozone (O3) flux, although not investigated in the present study, is inextricably linked with NOx exchange . Despite the fact that the interconversion of NOx and O3 is complex, deposition of NOx (i.e., resulting in low NO availability) limits the photochemical dissociation of O3. The resulting acceleration of the O3 concentration within the vegetation canopies is followed by soil and stomatal uptake, and possible damage to sensitive plant species .
Depending on the management strategy, it was observed that either relative humidity or precipitation along with soil/air temperature inversely regulated NOx effluxes. Biomass input by mulching might facilitate the immobilization of mineralized N, with the extent of this process presumably depending on the biochemical compositions of the added biomass (e.g., C/N ratio, lignin and polyphenol content; ). However, the short-term variation inherent to environmental conditions, including comparatively higher ambient NOx levels and edaphic conditions, might also be an important contributing factor influencing NOx emissions/deposition . Similarly, Hou and Tsuruta also reported a small efflux of NO during the post-harvest periods when using incorporated cabbage residues . Despite these findings, a strong relationship between NOx and both NH4+ and NO3- concentration has been reported elsewhere . High NH4+ concentrations could facilitate NO emissions under aerobic conditions, with nitrification being the dominant process given that it is typically not a major end-product of denitrification under anaerobic conditions or serves as a sink [44-46].
Finally, temperature is generally considered to be an important factor controlling NOx fluxes, subject to the availability of NH4+ or NO3-. Yet the influence of temperature might be negligible with either extremely wet or dry soils, which impede the diffusion of NOx . In our study, total NOx efflux was positively correlated with the combination of sunshine hours, precipitation, and the biomass N added during the two-week measurement periods, and with a high predictive level (87% or 95% depending on whether input variables from the mulching systems were included or not). However, the daily peaks for NOx generally followed precipitation events in agreement with observations of Hutchinson et al., who stated that the pulsing intensity and duration of NOx efflux depended on the size of the precipitation event . Even so, uncertainty still prevails as to the factors, like elevated atmospheric NOx levels, influencing its deposition. Indeed, the correlation analyses in this research indicated the dominancy of NOx emission over deposition once the weather conditions were favourable. However, measurements of NOx flux over longer periods are necessary before any conclusive remarks on the net annual rate of emissions/depositions occurring in the three systems studied here can be made.
Our results indicate that clover-grass species can use Biologically Fixed-N (BNF) to help meet their N requirements during re-growth periods following either mulching or removal after cutting. The mulched clover-grass might release larger amounts of NH3 into the atmosphere compared to the non-mulched clover-grass or mulched oilseed radish, and that could demonstrate remarkably low NH3 emissions under field conditions. It appears that the biochemical composition of the added plant material has a large influence on the total NH3 emissions, whereas sunshine hours and precipitation seem to determine the NOx fluxes instead. All three management systems under investigation could act as sources of NH3, but as sinks for NOx, with NH3 volatilization being the dominant process. Overall, the clover-grass farming practices could be more beneficial than those involving oilseed radish in temperate climates because clover-grass (i) could compensate for the application of inorganic N through BNF, (ii) demonstrated lower NH3 and NOx emissions/depositions, and (iii) could, as an N-rich material, either enrich the soil N pool for the succeeding crop cultivation through mulching or could be used as a forage product after cutting. Results imply that further long-term measurements are desirable, especially to gain insight into the annual exchange of unavoidable gaseous N between each of the ecosystems and the atmosphere. This is to assess air quality and offer technological and policy options to meet significant challenges facing by the world.
The Bavarian State Ministry for Agriculture and Forestry, Germany funded this research to Dr. Khalil also acknowledges Alexander von Humboldt Foundation, Germany for a research award and Department of Agriculture, Food and the Marine (DAFM), Ireland for funding a project that helped to complete this manuscript. We also thank Professor Olaf Bininda-Emonds for improving this manuscript.
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