Effect of fertilization on decomposition of 14C labelled plant residues and their incorporation into soil aggregates

Returning crop residues to soil helps to maintain soil C stocks. Organic C stocks and microbial biomass are important factors controlling the decomposition or retention of crop residues in soil and the formation of aggregates. Little is known about the specific contribution of crop residues to soil aggregate size fractions in the framework of long-term fertilization. This study investigated the effects of long-term fertilization on the decomposition of 14C-labelled plant residues and their incorporation into soil organic matter (SOM) of different aggregate size fractions. Soils were collected from 0–10 cm in the Ap horizon of a long-term (since 1988) field experiment at Grossbeeren (Germany). The following four fertilization treatments were used: 1) without fertilization or manuring (Control), 2) nitrogen applied by mineral fertilizer (N), and 3) manure with low (M) and 4) high (2 M) application doses. Soils were incubated for 100 days at 20 °C, with or without 14C-labelled plant residue. The incorporation of 14C into three aggregate size fractions–large macroaggregates (2–1 mm), small macroaggregates (1–0.25) and microaggregates (<0.25 mm)–was analyzed. 15 days of incubation, 44–57% of plant residue was mineralized in the order: M > N > control soil > 2 M. Adding plant residues increased soil β-glucosidase activity and microbial biomass C. On day 16 of incubation, more residue 14C was retained in small and large macroaggregates than in microaggregates in the control soil. In contrast, in fertilized soils the reverse was measured. Additionally, N, M and 2 M soils showed significant differences by incorporation of 14C in microbial biomass and β-glucosidase activity in different aggregate size fractions. The results imply that long-term fertilization significantly increased the residue 14C retention in microaggregate size fractions and its decomposition in soils.