Catherine J. Watson

Insights into the Effect of Soil pH on N2O and N2 Emissions and Denitrifier Community Size and Activity

ABSTRACT The objective of this study was to investigate how changes in soil pH affect the N2O and N2 emissions, denitrification activity, and size of a denitrifier community. We established a field experiment, situated in a grassland area, which consisted of three treatments which were repeatedly amended with a KOH solution (alkaline soil), an H2SO4 solution (acidic soil), or water (natural pH soil) over 10 months. At the site, we determined field N2O and N2 emissions using the 15N gas flux method and collected soil samples for the measurement of potential denitrification activity and quantification of the size of the denitrifying community by quantitative PCR of the narG, napA, nirS, nirK, and nosZ denitrification genes. Overall, our results indicate that soil pH is of importance in determining the nature of denitrification end products. Thus, we found that the N2O/(N2O + N2) ratio increased with decreasing pH due to changes in the total denitrification activity, while no changes in N2O production were observed. Denitrification activity and N2O emissions measured under laboratory conditions were correlated with N fluxes in situ and therefore reflected treatment differences in the field. The size of the denitrifying community was uncoupled from in situ N fluxes, but potential denitrification was correlated with the count of NirS denitrifiers. Significant relationships were observed between nirS, napA, and narG gene copy numbers and the N2O/(N2O + N2) ratio, which are difficult to explain. However, this highlights the need for further studies combining analysis of denitrifier ecology and quantification of denitrification end products for a comprehensive understanding of the regulation of N fluxes by denitrification.

Soil properties and the ability of the urease inhibitor N-(n-butyl) thiophosphoric triamide (nBTPT) to reduce ammonia volatilization from surface-applied urea

Abstract A laboratory study was made to evaluate the ability of the urease inhibitor N -(n-butyl) thiophosphoric triamide (nBTPT) to reduce NH 3 volatilization from surface-applied urea. Urea was amended with 0.0, 0.01, 0.058 or 0.28% nBTPT (w/w) and was applied, at a rate equivalent to 100kgN ha −1 , to the surface of 16 grassland soils, selected to show a wide range of different chemical and physical characteristics. Daily NH 3 loss was measured for 9 days, after fertilizer application, using ventilated enclosures at 13°C. The influence of soil properties on the effectiveness of nBTPT as a urea amendment was investigated using a modelling approach followed by stepwise multiple regression analysis. NH 3 volatilization from unamended urea ranged from 5.8 to 38.9% of the N applied and was greatest from a soil with a high pH and low titratable acidity. The % inhibition of NH3 loss by nBTPT was highly dependent on soil type, being effective on some soils even at the 0.01% nBTPT level. The % inhibition at the 0.28% level ranged from 99.4 to 54.4%. Modelling the relationship between total NH 3 loss and inhibitor concentration showed that the % nBTPT required to achieve a given % decrease in NH3 volatilization was constant for all soils. For example, 0.092% nBTPT was predicted to lower total NH3 loss by 90% from any given soil. The response to increasing inhibitor concentration in lowering NH 3 volatilization was greatest in a soil with low organic matter content and high pH. Stepwise multiple regression analysis showed that 4 soil properties (titratable acidity, pH-H 2 O, urease activity and cation-exchange capacity) contributed significantly to explaining 90.6% of the variation in total NH 3 loss. In all soils the % urea remaining at the end of the incubation was 2.6, 4.6, 10.9 and 25.5% for 0.00, 0.01, 0.058 and 0.28% nBTPT, respectively. The persistence of urea treated with nBTPT was particularly marked in soils with a high pH and could have physiological implications for plants. It is particularly interesting that nBTPT was most effective in soils which showed high NH 3 volatilization from unamended urea.

Effectiveness of the urease inhibitor NBPT (N-(n-butyl) thiophosphoric triamide) for improving the efficiency of urea for ryegrass production

In a field experiment ammonia volatilization and yield response were measured when calcium ammonium nitrate (CAN), urea or urea plus 0.5% w/w N-(n-butyl) thiophosphoric triamide (U + NBPT) were surface-applied to an established perennial ryegrass sward. NBPT lowered cumulative NH3 loss from ventilated enclosures over 13 days from 8.1% of the urea N applied to 1.9% and delayed, by approximately 5 days, the time at which maximum loss occurred. Ammonia volatilization from CAN was low being less than 0.1% of the N applied. However, actual NH3 volatilization loss rates were probably underestimated due to the low air exchange rates used in the ventilated enclosures.The relative efficiency of urea compared to CAN was 91.2% in terms of dry matter yield. Recovery of N by difference was 57.2% for urea compared with 68.7% for CAN. NBPT improved the yield performance of urea making the amended fertilizer comparable to that of CAN.

Confirmation of co-denitrification in grazed grassland

Pasture-based livestock systems are often associated with losses of reactive forms of nitrogen (N) to the environment. Research has focused on losses to air and water due to the health, economic and environmental impacts of reactive N. Di-nitrogen (N2) emissions are still poorly characterized, both in terms of the processes involved and their magnitude, due to financial and methodological constraints. Relatively few studies have focused on quantifying N2 losses in vivo and fewer still have examined the relative contribution of the different N2 emission processes, particularly in grazed pastures. We used a combination of a high 15N isotopic enrichment of applied N with a high precision of determination of 15N isotopic enrichment by isotope-ratio mass spectrometry to measure N2 emissions in the field. We report that 55.8 g N m−2 (95%, CI 38 to 77 g m−2) was emitted as N2 by the process of co-denitrification in pastoral soils over 123 days following urine deposition (100 g N m−2), compared to only 1.1 g N m−2 (0.4 to 2.8 g m−2) from denitrification. This study provides strong evidence for co-denitrification as a major N2 production pathway, which has significant implications for understanding the N budgets of pastoral ecosystems.

A review of nitrous oxide mitigation by farm nitrogen management in temperate grassland-based agriculture

Nitrous oxide (N2O) emission from grassland-based agriculture is an important source of atmospheric N2O. It is hence crucial to explore various solutions including farm nitrogen (N) management to mitigate N2O emissions without sacrificing farm profitability and food supply. This paper reviews major N management practices to lower N2O emission from grassland-based agriculture. Restricted grazing by reducing grazing time is an effective way to decrease N2O emissions from excreta patches. Balancing the protein-to-energy ratios in the diets of ruminants can also decrease N2O emissions from excreta patches. Among the managements of synthetic fertilizer N application, only adjusting fertilizer N rate and slow-released fertilizers are proven to be effective in lowering N2O emissions. Use of bedding materials may increase N2O emissions from animal houses. Manure storage as slurry, manipulating slurry pH to values lower than 6 and storage as solid manure under anaerobic conditions help to reduce N2O emissions during manure storage stage. For manure land application, N2O emissions can be mitigated by reducing manure N inputs to levels that satisfy grass needs. Use of nitrification inhibitors can substantially lower N2O emissions associated with applications of fertilizers and manures and from urine patches. N2O emissions from legume based grasslands are generally lower than fertilizer-based systems. In conclusion, effective measures should be taken at each step during N flow or combined options should be used in order to mitigate N2O emission at the farm level.

The response of grass for silage to sulphur application at 20 sites in Northern Ireland

Twenty field site3 were selected for their potential sulphur-deficient status. The effect of sulphur at 10 kg S/ha per cut as gypsum or kieserite on the yield and composition of grass for silage given intensive fertilizer was measured at two or three cuts in 1985. Other incidental sulphur inputs in P and K fertilizers and organic manures were minimized. There were significant increases (P Using soil analyses to predict sulphur-deficient sites had limited success. The conclusions from this study were that soils with extractable sulphate values 10 mg S/l had a 1 in 3 chance of being sulphur deficient. Using plant analyses to diagnose sulphur-deficient herbage had also limited success. In this study herbage with an N/S ratio > 14 was sulphur deficient while herbage with a ratio > 12 had a 1 in 2 chance of being deficient. The proportion of sulphur responsive sites in this study is an overestimate for Northern Ireland as a whole. Most soils in this country have higher clay and organicmatter contents than the field sites. The application of organic manures in normal agricultural practice is likely to be an important source of sulphur to grass for cutting.

Efficiency and future potential of urea for temperate grassland

The efficacy of urea as a grassland fertilizer under temperate conditions has been assessed in a wide variety of comparisons with either ammonium nitrate or calcium ammonium nitrate (CAN). Data from the British Isles have been evaluated and compared to results mainly from continental Europe.In general urea is as good as CAN early in the growing season, but less-effective in summer. There is no evidence to indicate that urea is significantly more variable that CAN for spring grass production. Maximum yields with urea are lower than those with CAN.The principal reason for inefficiency of urea is volatilization loss of ammonia. Chemodenitrification is also likely to be important but has not been quantified satisfactorily. In contrast, leaching and denitrification are the principal loss processes with ammonium nitrate. The translation of nitrogen uptake into dry matter yield may be less effective with urea than with CAN.A wide variety of strategies have potential for improving the efficiency of urea and these have been reviewed. Recent developments with urease inhibitors offer the promise of an effective compound in the near future. Such a development could lead to urea displacing ammonium nitrate as the dominant N fertilizer on temperate grassland.

The nitrification inhibitor dicyandiamide increases mineralization–immobilization turnover in slurry-amended grassland soil

Nitrification inhibitors are used in agriculture for the purpose of decreasing nitrogen (N) losses, by limiting the microbially mediated oxidation of ammonium (NH 4 + ) to nitrate (NO 3 − ). Successful inhibition of nitrification has been shown in numerous studies, but the extent to which inhibitors affect other N transformations in soil is largely unknown. In the present study, cattle slurry was applied to microcosms of three different grassland soils, with or without the nitrification inhibitor dicyandiamide (DCD). A solution containing NH 4 + and NO 3 − , labelled with 15 N either on the NH 4 + or the NO 3 − part, was mixed with the slurry before application. Gross N transformation rates were estimated using a 15 N tracing model. In all three soils, DCD significantly inhibited gross autotrophic nitrification, by 79–90%. Gross mineralization of recalcitrant organic N increased significantly with DCD addition in two soils, whereas gross heterotrophic nitrification from the same pool decreased with DCD addition in two soils. Fungal to bacterial ratios were not significantly affected by DCD addition. Total gross mineralization and immobilization increased significantly across the three soils when DCD was used, which suggests that DCD can cause non-target effects on soil N mineralization–immobilization turnover.

The influence of soil properties on the effectiveness of phenylphosphorodiamidate (PPD) in reducing ammonia volatilization from surface applied urea.

Urea can be an inefficient N source due to rapid hydrolysis by soil urease leading to NH3 volatilization. The current study investigated the effect of the urease inhibitor phenylphosphorodiamidate (PPD) incorporated at two concentrations (0.5% and 1% w/w) within the fertilizer granule on NH3 volatilization from surface applied urea. The daily rates of NH3 loss from 20 soils of widely differing properties from Northern Ireland were measured over 14 days using ventilated enclosures under simulated spring conditions. Cumulative loss rates were calculated and fitted to a logistic model from which total NH3 loss (Amax) and the time to maximum rate of loss (Tmax) were determined. Stepwise multiple linear regression analysis related the effectiveness of PPD in reducing NH3 volatilization from urea to soil properties.The total cumulative loss of ammonia from unamended urea varied from 0.37 to 29.2% depending on soil type. Ammonia volatilization appeared to be greatest on a soil with a high pH (R2 = 0.65), a low titratable acidity (TA) (R2 = 0.63) and a soil that was drying out (R2 = 0.50). Soil pH was negatively correlated with TA (r = −0.826, P < 0.001) suggesting that soils with a low TA may have received recent lime. Including cation exchange capacity (CEC) and % N as well as pH-KCl in the multiple linear regression equation explained 86% of the variance.The effectiveness of PPD in reducing Amax varied between 0% to 91% depending on soil type, the average over all 20 soils being 30 and 36% for 0.5% and 1% PPD respectively. The most important soil properties influencing the effectiveness of the urease inhibitor were soil pH-H2O and TA accounting for 33% and 29% of the variance respectively. PPD was less effective on a soil with a high pH and low TA. These were the soil conditions that led to high NH3 volatilization from unamended urea and may explain why PPD had limited success in reducing ammonia loss on these soils. Multiple linear regression analysis indicated that 75% of the variation in the % inhibition of NH3 loss by PPD could be significantly accounted for by pH-H2O, initial soil NO3--N concentration, % moisture content and % moisture loss.The delay in Tmax by PPD ranged from 0.19 to 7.93 days, the average over all 20 soils being 2.5 and 2.8 days for 0.5% and 1% PPD respectively. TA, % moisture content, urease activity and CEC were soil properties that significantly explained 83% of the variation in the % delay in Tmax by PPD in multiple linear regression analysis. However, none of these soil properties were significant on their own. As urea hydrolysis occurs rapidly in soil, delaying Tmax under field conditions would increase the chance of rain falling to move the urea below the soil surface and reduce NH3 volatilization. A urease inhibitor should be more effective than PPD on soils with a high pH and low TA to be successful in reducing high NH3 losses.

The comparative effects of ammonium nitrate, urea or a combined ammonium nitrate/urea granular fertilizer on the efficiency of nitrogen recovery by perennial ryegrass

The comparative effects of ammonium nitrate (AN), urea or a combined 1:1 (w/w) AN/urea granular fertilizer with two different fillers (CaCO3 or silica) were investigated on the efficiency of dry matter production and15N recovery by perennial ryegrass grown in pots under controlled environmental conditions.There was no significant difference between CaCO3 and silica as the filler and therefore no indication that the presence of CaCO3 in the pellet enhanced N loss from urea. Ammonium nitrate was the most efficient N source and urea the least efficient in terms of all the parameters studied. The15N budget in shoots, roots and soil indicated that only 60% of the nitrogen from urea was recovered at the end of the experiment compared with 95% for AN. However, the % recovery of15N from urea was increased by 17% in the presence of AN whereas the % recovery of AN was decreased by 19% in the presence of urea. The combined 1:1 (w/w) AN/urea source therefore gave intermediate yields between AN and urea alone. The results indicate that an interaction occurred between AN and urea in the granule.