Carbon sequestration efficiency in paddy soil and upland soil under long-term fertilization in southern China

Soil organic carbon (SOC) stock can be improved through the return of crop residues. However, the efficiency of C sequestered in soil (i.e., Delta SOC/Delta C input) might differ among crop systems. In this paper, we investigated the C input and SOC stabilization in paddy soil and upland soil under different long-term fertilization practices. Our objectives were to determine (i) the response of SOC stock to C input under different fertilization practices, and (ii) C sequestration efficiency in the two contrasting agroecosystems. The long-term fertilization experiment in paddy soil started in 1981, while the adjacent upland soil experiment commenced in 1986. Each experiment consists of 9 treatments: CK (no fertilization), N, P, K, NP, NK, NPK, 2NPK (double dose), and NPKOM (NPK plus organic manure). Physical SOC fractions (cPOM, silt + clay_f, fPOM, iPOM_m, silt + clay_m) were isolated by sieving, dispersion, and density flotation. Fertilization increased crop yield and C input, but it did not change the quality of SOC as revealed by CPMAS-C-13 NMR spectra. During the period of the experiment, SOC stock was improved by 6.7-15.3 Mg ha(-1) in paddy soil for all fertilization practices, while in upland soil the CK, N, P, K, and NP fertilizations reduced the SOC by 1.2-3.8 Mg C ha(-1) and the other four fertilizations increased it by 0.5-7.4 Mg C ha(-1). The change in SOC was mainly ascribed to the POM fraction, whereas the two silt + clay sized fractions were independent of fertilization practice except the NPKOM treatment. At a given C input, the C sequestration efficiency was greater in paddy soil than in upland soil, which may be attributed to lower microbial activity but greater chemical (i.e., oxalate-soluble Fe) and physical stabilizations (i.e., soil structure) in paddy field. Our results indicate that paddy soil may sequester more SOC, with higher efficiency, than upland soil does. (c) 2013 Elsevier B.V. All rights reserved.