Estimation of nitrous oxide emission from Hungarian semi-arid sandy and loess grasslands; effect of soil parameters, grazing, irrigation and use of fertilizer

Abstract

Nitrous oxide (N 2O) is emitted into the atmosphere in substantial quantities as an intermediate product of mainly denitrification processes and soil nitrification. N 2O emission from Hungarian sandy and loess type soils has been measured between August 2002 and December 2004. The effects of soil parameters and different farming activities (grazing exclusion, irrigation and mineral fertilization) on the rate of emission were investigated. Soil N 2O efflux was determined by measuring the accumulation of N 2O in 10 parallel small ( V = 400 cm 3, A = 80 cm 2) static chambers. Samples were taken with evacuated tubes followed by gas chromatography–mass spectrometry analysis. Emission from sandy grassland under extensive farming proved to be negligible below soil temperature of 5 °C, the mean fluxes at t soil > 10 °C and t soil < 10 °C were differing significantly (15.4 and 1.7 μg N m −2 h −1, respectively). N 2O efflux was linked to temperatures up to t soil = 20 °C, while at higher soil temperatures a decrease of efflux was observed. The optimum soil wetness (WFPS, water-filled pore space) for N 2O production was between 40 and 50% for this sandy soil. The annual sum of efflux for control plots in dry (2003) and wet (2004) years was almost the same (0.80 and 0.74 kg N ha −1 yr −1, respectively), probably due to the fact that in dry year soil wetness is lower than the optimum, and vice versa, in a wet year the high WFPS did not favoured N 2O production. Grazing (50–80 cows per km 2) enhanced emission by 4–18% in dry and wet years, respectively. Around 6% of total atmospheric deposition (13.8 kg N ha −1 yr −1) was transformed into N 2O and emitted into the atmosphere from the sandy soil. A two-year fertilization experiment above loess soil (August 2002–July 2004, with regular precipitation amount during the period) resulted in an emission factor of 0.26% as a consequence of applying 100 kg N ha −1 yr −1 ammonium nitrate mineral fertilizer in April, each year. Average fluxes were 1.13 and 0.90 kg N ha −1 yr −1 above fertilized and control plots, respectively. Monoliths from loess soil were used to study the effect of irrigation on N 2O fluxes. In contrast to sandy soil there were detectable soil fluxes below 5 °C. N 2O production increases with soil temperature until 15–20 °C above which lower emissions were detected. The optimum wetness for N 2O production in loess soil was around 30–40 WFPS but above 70–80% (up to saturation) other, yet unexplained maximum was observed. Irrigation enhanced the soil flux by 70% in the dry year (2003), when measured fluxes were 1.56 and 0.92 kg N ha −1 yr −1 on irrigated and control plots, respectively.