Effects of farmyard manure application and grassland renovation on net ecosystem carbon balance in a temperate grassland: analysis of 11-year eddy covariance data

Abstract

Grasslands provide a globally essential ecosystem service by storing carbon (C). Management practices affect the net C balance in grasslands positively and negatively. In Japan, farmyard manure (FYM) application and grassland renovation are common and essential grassland management practices; however, the long-term effects of these practices on C balance are not known. We conducted a field experiment in a managed grassland for 11 years (2004-2015) to investigate the effects of FYM application and grassland renovation on net ecosystem C balance (NECB), estimated from eddy covariance measurements. Two experimental plots were used: One received only chemical fertilizer (CF plot), and the other received FYM and supplementary chemical fertilizer (CM plot). Grassland renovation with full inversion tillage was conducted twice during the study period. The C annually removed from the grassland through harvest always exceeded the annual net ecosystem production, resulting in a net C loss (NECB =-1.32 Mg C ha-1 y-1) in the CF plot. In contrast, the CM plot gained net C (NECB = 1.79 Mg C ha-1 y-1) due to FYM application. The difference in ecosystem respiration between the plots suggested that 45% of C in annually applied FYM remained in the soil. The FYM application-mediated C input necessary to avoid net C loss was 3.67 Mg C ha-1 y-1. Grassland renovation conducted twice showed a mixed but relatively minor effect. Grassland renovation had little impact on annual NECB when the degree of sward/soil disturbance was low. In contrast, renovation with a high degree of sward/soil disturbance led to negative NECB in the year of renovation even in the CM plot. Overall, FYM application contributed to a net C gain, and the effect of grassland renovation on annual NECB was likely limited to the years of renovation.