Mineralization of 15N-labelled sheep manure in soils of different texture and water contents

In order to investigate the effect of soil moisture and texture on C and N mineralization of applied organic matter, sheep faeces was sandwiched between two halves of intact soil cores and incubated at 20°C. The soils contained 10.8 (L1), 22.4 (L3) and 33.7% (L5) clay, respectively, and were drained to seven different matric potentials in the range –15 to –1500 hPa. Evolution of CO2-C was determined during four weeks of incubation. Contents of NO3-N, 15N and microbial biomass N were determined at the end of the incubation. The net release of CO2-C from the faeces (estimated as the difference between soils with and without faeces) and the total CO2-C evolution from soils with faeces was not related to soil moisture contents. Most CO2-C evolved from faeces amended soils in the least clayey L1 soil. The faeces caused immobilization of soil NO3-N but the soil matric potential had no major effects on the net NO3-N production. Less than one per cent of the faeces 15N was found as NO3-N at the end of the incubation. The unamended sandy L1 soil held least N in microbial biomass but showed the largest increases in biomass N when faeces was added to this soil. The content of microbial biomass N in soils with faeces application peaked in the most clayey soil (L5). The recovery of faeces 15N at the end of the incubation ranged from 89 to 102%. The variation in 15N recovery was not related to soil clay content nor to soil matric potential. The experimental set-up was designed to mimic field conditions where manure is left as a discrete layer surrounded by structurally intact soil. In this situation the soil clay content and the soil moisture level appeared to have little influence on the C and N turnover in the manure layer