Soil Carbon and Nitrogen Dynamics Following Application of Pig Slurry for the 19th Consecutive Year I. Carbon Dioxide Fluxes and Microbial Biomass Carbon

Abstract

Agricultural soils often receive annual applications of manure for long periods. The objective of this study was to quantify the effects of 19 consecutive years of pig (Sus scrofa) slurry (PS) application on CO2 emissions and soil microbial biomass. Soil temperature, soil moisture, and extractable soil C were also determined to explain the variations in CO2 emissions and soil microbial biomass. Long‐term (19 yr) treatments were 60 (PS60) and 120 Mg ha−1 yr−1 (PS120) of pig slurry and a control receiving mineral fertilizers at a dose of 150 kg ha−1 yr−1 each of N, P2O5, and K2O. Very high CO2 emissions (up to 1.5 mg CO2 m−2 s−1) occurred during the first 2 d after PS application. Following that peak, decomposition of PS was rapid, with one‐half the total emissions occurring during the first week after slurry application. The rapid initial decomposition was exponential and was attributed to the decomposition of the labile fraction of the slurry C. The second phase was linear and much slower and probably involved more recalcitrant C material. Cumulative annual decomposition was proportional to the application rate, with 769 and 1658 kg C ha−1 lost from the 60 and 120 Mg ha−1 doses, respectively. Pig slurry application caused a rapid increase in soil microbial biomass (from ≈100 to up to 370 mg C kg−1 soil), which coincided with a peak in the concentration of extractable C and in CO2 emissions. Field estimates of the microbial specific respiratory activity suggested that the difference in soil respiration between the two slurry treatments was due to differences in the size of the induced microbial biomass rather than to differences in specific activity.