Turnover of Soil Nitrogen in a Semi-Arid Tropical Soil: From Basic Research to Knowledge Application in Organic Agriculture
Gunadhish Khanal1, Mariko Ingold1, Christine Wachendorf2, Andreas Buerkert1
1University of Kassel, Organic Plant Production and Agroecosystems Research in the Tropics and Subtropics, Germany
In the arid tropics, year-round high temperatures lead to fast mineralisation in irrigated agriculture which need quantification to develop management systems with enhanced nitrogen(N) use efficiency. In this context, the objective of this experiment was to investigate the loss of N2O-N from 15N labelled, soil incorporated manure in a laboratory incubation experiment. To this end male goats were fed 15N incorporated (0.675 at %) or normal (0.369 at %) Rhodes grass (Chloris gayana) to produce labelled (M) and unlabelled (m) manure. This manure was subsequently applied at a rate of 210 kg N ha-1 in experimental plots for two cropping seasons in 2013/14 and 2014/15. Soil samples labelled only in the first year (Mm), labelled in both the years (MM) and control soils (C) were collected at the end of the 2014/15 cropping season for the laboratory experiments. Four treatments, control (C), Mm + unlabeled manure (Mmm), MM + unlabeled manure (MMm) and MM + labelled manure (MMM) were replicated 4 times. 150 g soil was placed in air-tight 1.6-liter glass jars inside a thermostat chamber at 25°C for 31 days. The soil was reconditioned for 10 days (pre-incubation) at 50% water holding capacity before manure (equivalent to 100 kg N ha-1) was applied. Gas samples were taken 15 times during the incubation period before and after flushing the jars with fresh air. N2O and CO2 were analysed by gas chromatograph (GC) and isotopic N was determined by mass spectrometry (MS). Cumulative N2O-N emissions were 3.2, 12.1, 23.1 and 24.7 µg kg-1 soil for C, MMM, MMm and Mmm, respectively, for the total period. N derived from labelled manure (N2O-Nman) was lower in Mmm and higher in MMm and MMM. On the day of highest N2O emissions Nman was 9% for Mmm, 16% for MMm and 61% for MMM. During pre-incubation 7-15 % of N2O -N emission was derived from manure applied two years ago and 9-10% was derived from manure applied 1 year ago. Our study showed that even after two years, soil applied goat manure contributed to N2O emissions.
Keywords: 15N, N2O, Oman
Contact Address: Andreas Buerkert, University of Kassel, Organic Plant Production and Agroecosystems Research in the Tropics and Subtropics, Steinstraße 19, 37213 Witzenhausen, Germany, e-mail: buerkertuni-kassel.de