Mitigation potential of urease inhibitory compounds in reducing ammonia emissions from cattle urine in dairy-grazed pasture soils : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy (PhD) in Soil Science at Massey University, Palmerston North, New Zealand
The excretal deposition by grazing animals, especially urine containing about 80% urea, and urea fertiliser use on pastoral farms are major sources of ammonia (NH3) emissions in New Zealand (NZ) agriculture. Recent intensification of dairy farming in NZ has resulted in a substantial increase in the use of nitrogen (N) fertiliser, especially urea, and the quantity of urine deposited by grazing dairy cows onto pasture soils, and as a consequence, higher NH3 emissions. These emissions represent economic and environmental losses.
The urease inhibitor (UI) N-(n-butyl) thiophosphoric triamide (nBTPT) has shown effectiveness in reducing emissions when applied with fertiliser urea or cattle urine in NZ dairy-grazed pasture soils. However, the inhibitory effect of nBTPT on reducing NH3 emissions is effective for a relatively short period (7 - 14 days), during which emissions from urea fertiliser is inhibited. But, in the context of grazed pasture, to reduce the NH3 emitted from deposited urine following each grazing event, regular applications of nBTPT are required, which would be prohibitively expensive. To mitigate NH3 emissions from cattle urine deposited during more than a single grazing, in dairy-grazed pasture soils, it is necessary to identify alternative longer lasting inhibition approaches to using nBTPT. Therefore, the overall objective of this thesis was to assess the effectiveness and longevity of potentially longer-lasting non-specific inhibitors copper (Cu) and zinc (Zn), and the specific inhibitor N-(2-Nitrophenyl) phosphoric triamide (2-NPT) in reducing NH3 emissions following cattle urine applied to pasture soils.
The study presented in this thesis initially examined the influence of inherent and added Cu and Zn in inhibiting soil urease activity (UA), and the role of soil organic carbon (C) and soil textural and mineralogical properties on influencing the ability of these metals at inhibiting soil UA of dairy pasture soils under laboratory incubations. The study then evaluated the effect of the recently introduced UI 2-NPT on NH3 emissions, soil microbial biomass C, pasture dry matter yield and N uptake, which was compared with the more commonly used UI nBTPT. This study involved both laboratory and field experiments.
The first laboratory experiment assessed the effect of inherent and added Cu and Zn in inhibiting soil UA of dairy-grazed pasture soils. The results showed significant positive correlations between soil total C and N with soil UA for 23 soils from the Waikato region of NZ. However, there were no significant negative correlations between soil UA with inherent Cu and Zn levels. Similarly, the addition of Cu up to 20 mg kg⁻¹ soil and the combination of 5 mg Cu and 5 mg Zn kg⁻¹ soil did not significantly reduce soil UA of 4 dairy-grazed pasture soils, with contrasting organic C levels.
In the second laboratory experiment, the influence of the soil C factor (soil organic C, and other related soil properties, such as clay content and cation exchange capacity (CEC)) on the effectiveness of Cu and Zn to inhibit urea hydrolysis in soil supernatants were studied. When Cu was added to 2 different soil supernatants, at rates of 5, 10, and 20 mg Cu kg⁻¹ soil, there was a significant reduction in hydrolysis of urea applied at either 120 or 600 mg urea-N kg⁻¹ soil. Additions of Zn, at a rate of 20 mg kg⁻¹ soil achieved negligible or small reductions in urea hydrolysis after either 120 or 600 mg urea-N kg⁻¹ soil applications to soil supernatants. These results suggest that Cu has a urease inhibitory effect, but its ineffectiveness in C rich pasture soils is caused by reduced bioavailability as a result of high Cu complexation. However, Zn had a negligible inhibitory effect on soil UA at the rate used in this experiment. Overall these results support the conclusion that neither metal is likely to be a practical UI for reducing NH3 emissions from NZ dairy-grazed pasture soils.
The effectiveness and longevity of 2-NPT and nBTPT in reducing NH3 emissions from cattle urine applied to 2 dairy-grazed pasture soils were evaluated under laboratory conditions. The inhibitors were applied at the start of the experiment and urine was applied at 4 stages; (A) immediately before, (B) 29 days after, (C) 56 days after, and (D) 29 days after and again 60 days after inhibitor application, and NH3 emissions were measured following each urine application. There were 3 application rates of 2-NPT; 0.025, 0.050, and 0.075% of total urine-N applied at Stage-A, and one rate of nBTPT; 0.025% of total urine-N applied at Stage-A. The application depth of urine applied was 10 mm for Stages A, B and C and 7.2 mm for Stage-D. The % applied urine-N that was emitted as NH3 at the different stages ranged from 14.2 to 50.5% for the soils studied. Both UIs equally reduced total NH3 emissions (20.6 - 27.3%), from both of the soil types, when urine was applied immediately before inhibitor application. The inhibitor 2-NPT continued to reduce emissions (5.6 - 7.4%) from urine applied up to 56 days after the inhibitor application, but only for the soil with lower microbial biomass C and UA, suggesting that 2-NPT has slightly greater longevity of efficacy than nBTPT. When urine was applied immediately before inhibitor application, inhibitors had no effect on soil microbial biomass C, measured 31 days after inhibitor application, which suggest specificity of UIs on inhibiting UA.
Two field experiments were conducted during summer and autumn to assess whether the differences observed between the inhibitors 2-NPT and nBTPT in the laboratory experiment are also achieved under field conditions. In the summer experiment, the inhibitors were applied at the start of the experiment and urine was applied at 3 stages; (A) 3 hrs before, (B) 28 days after, and (C) 68 days after inhibitor application, and NH3 emissions were measured following each urine application. The application rates of inhibitors to the urine treatments for all 3 stages were based on the percentage (0.025%) of total urine-N applied at Stage-A. In the autumn experiment, urine was only applied either immediately before or 3 hrs before inhibitor application, also at a rate of 0.025% of total urine-N. The application depth of urine applied in both of the summer and autumn experiments was 10 mm. The NH3-N emitted in the summer experiment was between 15.3 - 23.6% of the applied urine-N (equivalent to 111 - 142 kg N ha⁻¹), however, in autumn the emissions were only 4.5% (equivalent to 27 kg N ha⁻¹ of the total N applied. In the summer experiment, only 2-NPT significantly reduced total NH3 emissions (19.5% reduction), which was only when urine was applied 28 days after the inhibitor application (Stage B). Both inhibitors significantly reduced emissions in autumn when urine was applied either 3 hrs before or immediately before inhibitor application. However, the effectiveness was greater when urine was applied immediately before inhibitor application (52.3 - 72.7% reduction) compared to urine applied 3 hrs before inhibitor application (35.0 - 41.2% reduction). The reduction was greater with 2-NPT (72.7% reduction) compared to nBTPT (52.3% reduction) when urine was applied immediately before inhibitor application. The field study confirmed the findings of laboratory study that the effectiveness and longevity of 2-NPT, in reducing NH3 emissions from cattle urine applied to pasture soils, is greater compared to nBTPT. There was no effect of inhibitors on pasture dry matter yield and N uptake in either of the field experiments.
The 2-NPT applied up to about a month prior to a grazing event in summer reduced NH3 emissions from urine patch areas at a rate equivalent to 26 kg N ha⁻¹ (based on 19.5 % reduction in summer) at the subsequent grazing. If 3 applications of 2-NPT are applied in summer (period when emissions are typically the highest) following 3 grazings that would reduce losses by 78 kg N ha⁻¹ from 3 subsequent grazings. When these reductions are extrapolated to determine the overall benefit on whole paddock basis, the total reduction in N loss (26 kg N ha⁻¹ x 3% x 3 grazings) is 2.3 kg N ha⁻¹ yr⁻¹, assuming the urine patches cover 3% of the grazed area per grazing. Thus, overall benefit from using 2-NPT is greater than nBTPT in reducing NH3 emissions from cattle urine deposited in dairy-grazed pastures. However, the size of the reduction from using 2-NPT on whole paddock was low, compared to the amount of N cycling in grazed pastures annually.
To further improve the effectiveness of inhibitors, applied after urine application in summer, future research could focus on enhancing the contact between the inhibitor and urine urea by increasing the volume of inhibitor used (>800 L ha⁻¹) and/or by implementing shorter durations (<3 hrs) between urine and inhibitor application. Further changes in volume of inhibitor applied and timing of inhibitor application should consider cost involved and feasibility in dairy farms with the current technology.