A 3-year assessment of nitrous oxide emission factors for urine and dung of grazing sheep in a subtropical ecosystem

Purpose

Grazing livestock has strong impact on global nitrous oxide (N₂O) emissions by providing N sources through excreta. The scarcity of information on factors influencing N₂O emissions from sheep excreta in subtropical ecosystems such as those of Southern Brazil led us to conduct field trials in three different winter pasture seasons on an integrated crop–livestock system (ICL) in order to assess N₂O emission factors (EF-N₂O) in response to variable rates of urine and dung.

Materials and methods

The equivalent urine-N loading rates for the three winter seasons (2009, 2010, and 2013) ranged from 96 to 478 kg ha^?1, and the dung-N rates applied in 2009 and 2010 were 81 and 76 kg ha^?1, respectively. Air was sampled from closed static chambers (0.20 m in diameter) for approximately 40 days after excreta application and analyzed for N₂O by gas chromatography.

Results and discussion

Soil N₂O-N fluxes spanned the ranges 4 to 353 μg m^?2 h^?1 in 2009, ??47 to 976 μg m^?2 h^?1 in 2010, and 46 to 339 μg m^?2 h^?1 in 2013. Urine addition resulted in N₂O-N peaks within for up to 20–30 days after application in the 3 years, and the strength of the peaks was linearly related to the N rate used. Emission factors of N₂O (EF-N₂O, % of N applied that is emitted as N₂O) of urine ranged from 0.06 to 0.34% and were essentially independent of N rate applied. By considering a ratio of N excreted by urine and dung of 60:40, a single combined excretal EF-N₂O of 0.14% was estimated.

Conclusions

Our findings showed higher mean EF-N₂O for sheep urine than that for dung (0.21% vs 0.03%), irrespective of the occurrence or not of urine patches overlap. This value is much lower than default value of 1% of IPCC’s Tier 1 and reinforces the needs of its revision.

     

Phosphorus adsorption and desorption in undisturbed samples from subtropical soils under conventional tillage or no‐tillage

The dynamics of P in soil is greatly influenced by the adsorption of phosphate onto Fe‐oxides. Access of phosphate to the surface of these minerals depends on the degree of soil aggregation, which in turn is influenced by soil management system. The primary purpose of this work was to investigate P adsorption and desorption in undisturbed and disturbed soil samples from an Ultisol (Rhodic Paleudult) and an Oxisol (Humic Hapludox) under conventional tillage (CT) or no‐tillage (NT). Phosphorus adsorption and desorption in undisturbed soil was studied by using a continuous flux system containing a P solution for adsorption measurements or deionized water and Mehlich‐I solution for desorption measurements. The Oxisol, which had higher clay, hematite, and goethite contents than the Ultisol, exhibited the highest maximum P adsorption capacity (P_max) values in disturbed samples. Also, the disturbed Ultisol samples had lower P_max values under NT than under CT. The undisturbed soil samples exhibited no significant differences in P adsorption between soil management systems, but P desorption was more marked under NT than under CT. The samples of Oxisol under NT exhibited lower P adsorption rates and higher P desorption rates than the CT samples of the same soil. The decreased P adsorption in undisturbed samples relative to disturbed samples suggests that P adsorption is influenced by physical properties of soil.     

Organic N forms of a subtropical Acrisol under no-till cropping systems as assessed by acid hydrolysis and solid-state NMR spectroscopy

This study was conducted to investigate the influence of land-use systems (grassland and cropland) and of long-term no-till cropping systems [bare soil, oat/maize (O/M), pigeon pea+maize (P+M)] on the composition of organic N forms in a subtropical Acrisol. Soil samples collected from the 0- to 2.5-cm layer in the study area (Eldorado do Sul RS, Brazil) were submitted to acid hydrolysis and cross-polarization magic angle spinning (CPMAS) ¹⁵N and ¹³C nuclear magnetic resonance (NMR) spectroscopies. The legume-based cropping system P+M contained the highest contents of non-hydrolysable C and N, hydrolysable C and N, amino acid N and hydrolysed unknown N. The relative proportion of non-hydrolysable N was higher in bare soil (30.0%) and decreased incrementally in other treatments based on the total C and N contents. The amino acid N corresponded to an average of 37.2% of total N, and was not affected by land use and no-till cropping systems. The non-hydrolysable residue contained lower O-alkyl and higher aromatic C concentrations, as revealed by CPMAS ¹³C NMR spectroscopy, and higher C:N ratio than the bulk soil. No differences in the bulk soil organic matter composition could be detected among treatments, according to CPMAS ¹³C and ¹⁵N NMR spectra. In the non-hydrolysable fraction, grassland showed a lower concentration of aromatic and a higher concentration of alkyl C than other treatments. From CPMAS ¹⁵N NMR spectra, it could be concluded that amide N from peptide structures are the main organic N constituent. Amide structures are possibly protected through encapsulation into hydrophobic sites of organic matter and through organomineral interaction.