Effects of fertilizer type and rate on labile soil fractions of a sandy Cambisol—long‐term and short‐term dynamics

The application of density fractionation is an established technique, but studies on short‐term dynamics of labile soil fractions are scarce. Objectives were (1) to quantify the long‐term and short‐term dynamics of soil C and N in light fraction (LFOC, LFON, ρ ≤ 2.0 g cm^–3) and microbial biomass C (C_mic) in a sandy Cambisol as affected by 28 y of different fertilization and (2) to determine the incorporation of C₄‐C into these labile fractions during one growing season of amaranth. The treatments were: straw incorporation plus application of mineral fertilizer (MSI) and application of farmyard manure (FYM) each at high (MSI_H, FYM_H, 140–150 kg N ha^–1 y^–1) and low (MSI_L, FYM_L, 50–60 kg N ha^–1 y^–1) rates at four field replicates. For all three sampling dates in 2008 (March, May, and September), stocks of LFOC, LFON and C_mic decreased in the order FYM_H > FYM_L > MSI_H, MSI_L. However, statistical significance varied markedly among the sampling dates, e.g., with LFOC being significantly different (p ≤ 0.05) in the order given above (sampling date in March), significantly different depending on the fertilizer type (May), or nonsignificant (September). The high proportion of LFOC on the stocks of soil organic C (45% to 55%) indicated the low capacity of soil‐organic‐matter stabilization on mineral surfaces in the sandy Cambisol. The incorporation of C₄‐C in the LFOC during one growing season of amaranth was small in all four treatments with C₄‐LFOC ranging from 2.1% to 3.0% of total LFOC in March 2009, and apparent turnover times of C₃‐derived LFOC ranged from 21 to 32 y for the sandy soils studied. Overall, our study indicates that stocks of LFOC and LFON in a sandy arable soil are temporarily too variable to obtain robust significant treatment effects of fertilizer type and rate at common agricultural practices within a season, despite the use of bulked six individual cores per plot, a common number of field replicates of four, and a length of treatments (28 y) in the order of the turnover time (21–32 y) of C₃‐derived LFOC.