Effect of grassland ploughing and reseeding on CO2 emissions and soil carbon stocks

Ploughing of grassland for reseeding can result in a temporary decline in soil carbon stocks due to reduced gross primary production and increased soil respiration. To improve understanding of processes affecting the decay of soil organic matter after grassland renovation we measured soil respiration and soil carbon stocks for several months after ploughing and reseeding of 16-year-old grass-clover swards on a sandy loam soil in Sep 2010 (I), May 2011 (II), Sep 2011 (III) and May 2012 (IV). Simultaneously, to establish the CO2-flux baseline, the ecosystem respiration, consisting of autotrophic and heterotrophic soil respiration, was measured on intact control swards (IG). Collected data were used to calibrate a soil carbon model to estimate mid- and long-term carbon losses. Results showed that, on the basis of the model used, heterotrophic soil respiration from ploughed plots exceeded the IG baseline in all experimental periods (I-IV). The amount of residual plant material at the time of ploughing and increased decay of native soil organic matter for approximately six months after soil disturbance were identified as the main drivers for enhanced soil respiration in the short-term. Long-term simulation of the calibrated model for 100 years indicated that net soil carbon stocks can decrease by 21 and 14 Mg C ha−1, compared with intact grassland, when sward renovation by reseeding, involving ploughing, is conducted every 5 to 10 years, and that grassland reseeding in spring carries a higher risk of soil carbon losses compared with reseeding later in the year.