J. W. van Groenigen

Dissolved Organic Nitrogen: An Overlooked Pathway of Nitrogen Loss from Agricultural Systems?

Conventional wisdom postulates that leaching losses of N from agriculture systems are dominated by NO(3)(-). Although the export of dissolved organic nitrogen (DON) into the groundwater has been recognized for more than 100 yr, it is often ignored when total N budgets are constructed. Leaching of DON into stream and drinking water reservoirs leads to eutrophication and acidification, and can pose a potential risk to human health. The main objective of this review was to determine whether DON losses from agricultural systems are significant, and to what extent they pose a risk to human health and the environment. Dissolved organic N losses across agricultural systems varied widely with minimum losses of 0.3 kg DON ha(-1)yr(-1) in a pasture to a maximum loss of 127 kg DON ha(-1)yr(-1) in a grassland following the application of urine. The mean and median values for DON leaching losses were found to be 12.7 and 4.0 kg N ha(-1)yr(-1), respectively. On average, DON losses accounted for 26% of the total soluble N (NO(3)(-) plus DON) losses, with a median value of 19%. With a few exceptions, DON concentrations exceeded the criteria recommendations for drinking water quality. The extent of DON losses increased with increasing precipitation/irrigation, higher total inputs of N, and increasing sand content. It is concluded that DON leaching can be an important N loss pathway from agricultural systems. Models used to simulate and predict N losses from agricultural systems should include DON losses.

Seasonal variation in N2O emissions from urine patches: effects of urine concentration, soil compaction and dung

Urine patches in pastures rank among the highest sources of the greenhouse gas nitrous oxide (N2O) from animal production systems. Previous laboratory studies indicate that N2O emissions for urine-N in pastures may increase with a factor five or eight in combination with soil compaction and dung, respectively. These combinations of urine, compaction and dung occur regularly in pastures, especially in so-called camping areas. The aims of this study were (i) to experimentally quantify the effect of compaction and dung on emission factors of N2O from urine patches under field conditions; (ii) to detect any seasonal changes in emission from urine patches; and (iii) to quantify possible effects of urine concentration and -volume. A series of experiments on the effects of compaction, dung, urine-N concentration and urine volume was set up at a pasture on a sandy soil (typic Endoaquoll) in Wageningen, the Netherlands. Artificial urine was applied 8 times in the period August 2000–November 2001, and N2O emissions were monitored for a minimum of 1 month after each application. The average emission factor for urine-only treatments was 1.55%. Over the whole period, only soil compaction had a clear significant effect, raising the average N2O emissions from urine patches from 1.30% to 2.92% of the applied N. Dung had no consistent effect; although it increased the average emissions from 1.60% to 2.82%, this was clearly significant (P< 0.01) for only one application date and marginally significant (P=0.054) for the whole experiment. Both compaction and dung increased water-filled pore space (WFPS) of the topsoil for a more prolonged time than high urine volumes. No effect of amount of urine-N or urine volume on N2O emissions relative to added N was detected for the whole experiment. There were clear differences between application dates, with highest emissions for urine-only treatments of 4.25% in October, 2000, and lowest of −0.11% in June, 2001. Emissions peaked at 60–70% WFPS, and decreased rapidly with both higher and lower WFPS. We conclude that compaction leads to a considerable increase in the N2O emissions under field conditions, mainly through higher WFPS. Dung addition may have the same effect, although this was not consistent throughout our experiment. Seasonal variations seemed mainly driven by differences in WFPS. Based on this study, mitigation strategies should focus on minimizing the grazing period with wet conditions leading to WFPS > 50%, avoiding camping areas in pastures, and on avoiding grazing under moist soil conditions. Greenhouse gas budgets for grazing conditions should include the effects of soil compaction and dung to represent actual emissions.

Bioenergy by-products as soil amendments? Implications for carbon sequestration and greenhouse gas emissions

An important but little understood aspect of bioenergy production is its overall impact on soil carbon (C) and nitrogen (N) cycling. Increased energy production from biomass will inevitably lead to higher input of its by‐products to the soil as amendments or fertilizers. However, it is still unclear how these by‐products will influence microbial transformation processes in soil, and thereby its greenhouse gas (GHG) balance and organic C stocks. In this study, we assess C and N dynamics and GHG emissions following application of different bioenergy by‐products to soil. Ten by‐products were selected from different bioenergy chains: anaerobic digestion (manure digestates), first generation biofuel by‐products (rapeseed meal, distilled dried grains with solubles), second‐generation biofuel by‐products (nonfermentables from hydrolysis of different lignocellulosic materials) and pyrolysis (biochars). These by‐products were added at a constant N rate (150 kg N ha−1) to a sandy soil and incubated at 20 °C. After 60 days, >80% of applied C had been emitted as CO2 in the first‐generation biofuel residue treatments. For second‐generation biofuel residues this was approximately 60%, and for digestates 40%. Biochars were the most stable residues with the lowest CO2 loss (between 0.5% and 5.8% of total added C). Regarding N2O emissions, addition of first‐generation biofuel residues led to the highest total N2O emissions (between 2.5% and 6.0% of applied N). Second‐generation biofuel residues emitted between 1.0% and 2.0% of applied N, with the original feedstock material resulting in similar N2O emissions and higher C mineralization rates. Anaerobic digestates resulted in emissions <1% of applied N. The two biochars used in this study decreased N2O emissions below background values. We conclude that GHG dynamics of by‐products after soil amendment cannot be ignored and should be part of the lifecycle analysis of the various bioenergy production chains.

Global trends and uncertainties in terrestrial denitrification and N2O emissions

Soil nitrogen (N) budgets are used in a global, distributed flow-path model with 0.5° × 0.5° resolution, representing denitrification and N2O emissions from soils, groundwater and riparian zones for the period 1900–2000 and scenarios for the period 2000–2050 based on the Millennium Ecosystem Assessment. Total agricultural and natural N inputs from N fertilizers, animal manure, biological N2 fixation and atmospheric N deposition increased from 155 to 345 Tg N yr−1 (Tg = teragram; 1 Tg = 1012 g) between 1900 and 2000. Depending on the scenario, inputs are estimated to further increase to 408–510 Tg N yr−1 by 2050. In the period 1900–2000, the soil N budget surplus (inputs minus withdrawal by plants) increased from 118 to 202 Tg yr−1, and this may remain stable or further increase to 275 Tg yr−1 by 2050, depending on the scenario. N2 production from denitrification increased from 52 to 96 Tg yr−1 between 1900 and 2000, and N2O–N emissions from 10 to 12 Tg N yr−1. The scenarios foresee a further increase to 142 Tg N2–N and 16 Tg N2O–N yr−1 by 2050. Our results indicate that riparian buffer zones are an important source of N2O contributing an estimated 0.9 Tg N2O–N yr−1 in 2000. Soils are key sites for denitrification and are much more important than groundwater and riparian zones in controlling the N flow to rivers and the oceans.

Diet effects on urine composition of cattle and N 2 O emissions

Ruminant production contributes to emissions of nitrogen (N) to the environment, principally ammonia (NH3), nitrous oxide (N2O) and di-nitrogen (N2) to air, nitrate (NO3 -) to groundwater and particulate N to surface waters. Variation in dietary N intake will particularly affect excretion of urinary N, which is much more vulnerable to losses than is faecal N. Our objective is to review dietary effects on the level and form of N excreted in cattle urine, as well as its consequences for emissions of N2O. The quantity of N excreted in urine varies widely. Urinary N excretion, in particular that of urea N, is decreased upon reduction of dietary N intake or an increase in the supply of energy to the rumen microorganisms and to the host animal itself. Most of the N in urine (from 50% to well over 90%) is present in the form of urea. Other nitrogenous components include purine derivatives (PD), hippuric acid, creatine and creatinine. Excretion of PD is related to rumen microbial protein synthesis, and that of hippuric acid to dietary concentration of degradable phenolic acids. The N concentration of cattle urine ranges from 3 to 20 g/l. High-dietary mineral levels increase urine volume and lead to reduced urinary N concentration as well as reduced urea concentration in plasma and milk. In lactating dairy cattle, variation in urine volume affects the relationship between milk urea and urinary N excretion, which hampers the use of milk urea as an accurate indicator of urinary N excretion. Following its deposition in pastures or in animal houses, ubiquitous microorganisms in soil and waters transform urinary N components into ammonium (NH4 +), and thereafter into NO3 - and ultimately in N2 accompanied with the release of N2O. Urinary hippuric acid, creatine and creatinine decompose more slowly than urea. Hippuric acid may act as a natural inhibitor of N2O emissions, but inhibition conditions have not been defined properly yet. Environmental and soil conditions at the site of urine deposition or manure application strongly influence N2O release. Major dietary strategies to mitigating N2O emission from cattle operations include reducing dietary N content or increasing energy content, and increasing dietary mineral content to increase urine volume. For further reduction of N2O emission, an integrated animal nutrition and excreta management approach is required.

The 18O signature of biogenic nitrous oxide is determined by O exchange with water

To effectively mitigate emissions of the greenhouse gas nitrous oxide (N(2)O) it is essential to understand the biochemical pathways by which it is produced. The (18)O signature of N(2)O is increasingly used to characterize these processes. However, assumptions on the origin of the O atom and resultant isotopic composition of N(2)O that are based on reaction stoichiometry may be questioned. In particular, our deficient knowledge on O exchange between H(2)O and nitrogen oxides during N(2)O production complicates the interpretation of the (18)O signature of N(2)O.Here we studied O exchange during N(2)O formation in soil, using a novel combination of (18)O and (15)N tracing. Twelve soils were studied, covering soil and land-use variability across Europe. All soils demonstrated the significant presence of O exchange, as incorporation of O from (18)O-enriched H(2)O into N(2)O exceeded their maxima achievable through reaction stoichiometry. Based on the retention of the enrichment ratio of (18)O and (15)N of NO(3)(-) into N(2)O, we quantified O exchange during denitrification. Up to 97% (median 85%) of the N(2)O-O originated from H(2)O instead of from the denitrification substrate NO(3)(-).We conclude that in soil, the main source of atmospheric N(2)O, the (18)O signature of N(2)O is mainly determined by H(2)O due to O exchange between nitrogen oxides and H(2)O. This also challenges the assumption that the O of N(2)O originates from O(2) and NO(3)(-), in ratios reflecting reaction stoichiometry.

Gaseous nitrogen emissions from livestock farming systems

Publisher Summary This chapter discusses the origin and controlling factors of gaseous nitrogen emissions, the uncertainty in the estimates, and possible measures that may be taken to decrease these emissions. The uncertainty in the estimated contributions of livestock farming systems to the total emissions of NH 3 , NO and N 2 O into the atmosphere stems in part from the paucity in measurement data of gaseous N losses from animal housing systems and manure storage systems, especially also for livestock farming systems in the developing countries. The number of farm animals is larger in developing countries than in developed countries, while the number of measurements of the emissions of NH 3, NO, N 2 O and N 2 from livestock farming systems is much larger in developed countries than in developing countries. Further, there are more data about N losses associated with the application of slurry and manure to agricultural land than about N losses from animal housing systems and manure storage systems, while N losses from housing systems, manure storage systems and from slurry application to land may be equally large.

Nitrous oxide emissions from multiple combined applications of fertiliser and cattle slurry to grassland

Fertiliser and manure application are important sources of nitrous oxide (N2O) emissions from agricultural soils. The current default IPCC emission factor of 1.0% is independent of the type of fertiliser and manure, and application time, method and rate. However, in the IPCC Tiered system it is possible to use more specific emission factors that better reflect the actual fertiliser and manure management in a given country or region. The first and primary aim of this study was to determine whether the combination of cattle slurry injection with fertiliser application, which is common practice in intensively managed grasslands in the Netherlands and neighbouring countries, warrants an adjusted emission factor. A second aim was to evaluate whether alternative emission factors, based on N uptake and N surplus, respectively, give more insight in the N2O emission rates of various fertilisation strategies. In a 2-year field experiment on sandy soil in the Netherlands we measured the annual N2O emission from grasslands receiving repeated simultaneous applications of fertiliser and cattle slurry. The N2O fluxes and N uptake by grass were measured from plots receiving calcium ammonium nitrate (CAN) at four application rates, either with or without additional application of liquid cattle slurry, applied through shallow soil injection. The average emission factor for fertiliser-only treatments was 0.15%. The annual N2O emissions were similar for treatments receiving only fertiliser or only cattle slurry. In the first experimental year, application of cattle slurry increased the emission factor for fertiliser to 0.35%, but the second year showed no effect of cattle slurry on the emission from fertiliser. With regard to the first objective, we conclude that these results do not conclusively justify an adjusted emission factor for combined application of fertiliser and cattle slurry. To minimise risks however, it is sensible to avoid simultaneous application of fertiliser and cattle slurry. The N2O emission factor expressed as percentage of kg N uptake by grass was consistently higher after combined application of fertiliser and cattle slurry (0.29%), compared to fertiliser-only (0.17%). With regard to the second objective we conclude that an emission factor based on N uptake expresses the relatively inefficient N supply of cattle slurry to crop growth better than the traditional emission factor based on N application.

Source determination of nitrous oxide based on nitrogen and oxygen isotope tracing dealing with oxygen exchange.

Source determination of nitrous oxide (N(2)O) from soils has so far been complicated by methodological constraints: the frequently used (15)N tracer method could not differentiate between pathways related to nitrification, that is, nitrifier nitrification (NN), nitrifier denitrification (ND), and nitrification-coupled denitrification (NCD). To overcome this problem, a dual isotope method using both (15)N and (18)O was proposed. However, O exchange between nitrogen oxides and water has been found to disturb such a method. We here explain in detail a novel dual isotope method that allows to quantify O exchange in denitrification and to differentiate N(2)O production from NN, ND, NCD, and fertilizer denitrification (FD). The method has already been applied to a range of soils with good success. Potential of and scope for further improvement of the method are discussed.