Saturday, 15 July 2006

The role of nitrification inhibitors in mitigating nitrogen and cation losses in grazed pasture.

Jagrati Singh1, Nanthi Bolan1, and Surinder Saggar2. (1) Soil and Earth Sciences, Turitea, Massey University, Palmerston North, New Zealand, (2) Landcare Research, Private Bag 11052, Palmerston North, New Zealand

The role of nitrification inhibitors in mitigating nitrogen and cation losses in grazed pasture.

Grasslands worldwide occupy 11700 million ha of vegetated lands and provide forage for over 1800 million livestock units and wildlife. These managed grasslands are highly productive, with increased pasture production being the major goal for the pastoral farmers for higher per hectare animal productivity. The fertility of grassland soils can be substantially altered by grazing animals mainly through the deposition of dung and urine and their subsequent transformation and transport in soils. Nitrogen (N) deposited in the grazed pastures, in the form of urine from dairy cows, where extremely heavy rates of N (up to 1000 kg N/ha) are applied in small “patches”, is a major environmental concern in New Zealand. This deposited nutrient is far in excess of immediate plant requirements and hence is susceptible to losses as ammonia (NH3) volatilization, nitrous oxide (N2O) emissions and removal to drainage water through nitrate (NO3-) leaching. Grazed pastures are identified as an important source of NH3 and N2O, which are implicated in acid rain, ozone depletion and global warming (i.e., greenhouse gas). Moreover, an increase in nitrate concentration in groundwater resulting from leaching has been linked to increasing incidences of NO3- toxicity in human and livestock (i.e. methaemoglobinaemia). Nitrate leaching is one of the major issues facing the New Zealand agriculture sector at present, when leached; NO3- also carries basic cations, which represents an economical loss for the farmer. Various approaches have been attempted to mitigate the economic and environmental impacts of N losses. Recently in New Zealand there has been increasing interest in the use of commercially formulated NIs (e.g., EcoN, N-Care) to reduce the loss of N through leaching and gaseous emissions, and enhance plant productivity. The study was conducted to examine the potential role of nitrification inhibitors (NI) to mitigate these losses. Nitrification inhibitors specifically retard the oxidation of ammonium (NH4+) to nitrite (NO2-), thereby decreasing the concentration of nitrate (NO3-) and the subsequent leaching and N2O emission. In this study a recently developed closed chamber technique that provides measurements of NH3 and N2O from urine treated intact soil-pasture cores was used. At the end of the experiment, soil cores were leached with 2.5 pore volumes of water and the leachate was analysed for basic cations (K+, Ca+2, Mg+2 ) and NO3-.

It was found that the addition of NI (DCD) @ 25 kg/ha to soil cores treated with different levels of urine (200, 400 and 800 kg N/ha) reduced the amount of N2O emission by 32 %, 54% and 78 %, respectively. Similarly, NO3- leaching decreased by 60-65% with the addition of DCD to the urine amended soil. There was significant reduction in the amount of cations leached with addition of DCD. However, there was increase in the concentration of NH4+ ions in the urine-amended soil with the addition of DCD which was liable to leaching.

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