Accumulation of nitrogen and microbial residues during 2000 years of rice paddy and non-paddy soil development in the Yangtze River Delta, China

Abstract

Lowland rice paddy soils may accumulate significant amounts of organic matter. Our aim was to investigate the role of prolonged paddy management on the nitrogen (N) status of the soils, and to elucidate the contribution of bacteria and fungi to long-term N accumulation processes. For this purpose, we sampled a chronosequence of 0-2000 years of rice cropping with adjacent non-paddy systems in the Bay of Hangzhou, China. The samples were analyzed for bulk density, total, mineral and microbial N (N mic ), and amino sugars as markers for microbial residues. The results showed that during the first 100 years of land embankment, both paddy and non-paddy soils accumulated N at a rate of up to 61 and 77 kg ha À1 per annum, reaching steady-state conditions after 110-172 years, respectively. Final N stocks in paddy fields exceeded those of the non-paddies by a factor of 1.3. The contribution of amino sugars to total N increased to a maximum of 34 g N kg À1 N in both land-use systems, highlighting a significant accumulation of N in microbial residues of the surface soils. Correspondingly, the ratio of N mic to microbial residue-N decreased to a constant value. In the paddy subsoils, we found that bacterial residues particularly contributed to the pool of microbial residue-N. Nevertheless, the absolute contents of amino sugars in paddy subsoils decreased during the last 1700 years of the chronosequence. We conclude that under paddy cultivation, soil microorganisms may accumulate parts of this N in their residues despite low overall N availability. However, this N accumulation is limited to initial stages of paddy soil development and restricted to the surface horizons, thus challenging its sustainability with future land-use changes.

Quantitative assessment of microbial necromass contribution to soil organic matter

“…Prior studies showed that the total content of soil amino sugar‐N in most instances significantly exceeded the total amount of soil microbial biomass‐N (Appuhn & Joergensen, ; Joergensen, ; Joergensen, Mäder, & Fließbach, ). In a chronosequence of prolonged rice paddy management, the concentration of nitrogen in soil amino sugars exceeded the nitrogen content in living biomass by a factor of 2 in plots with continuous cultivation of over 190–2,000 years (Roth et al, ). The constancy of this factor over such a long period of time suggests that the amino sugar levels roughly mirror an apparent biologic steady state between production and degradation of microbe‐derived materials.…”

The carbon count of 2000 years of rice cultivation

“…It was not possible to find any soil older than 700 years not used for rice cultivation. Traditional paddy management in the region is a rotation with rice grown during the wet season, followed by wheat or another upland crop during the dry season (Fan et al, 2005;Roth et al, 2011). We cannot assume a constant management at all the sites during the 2000 years of paddy soil development.…”

Response of Vertisols, Andosols, and Alisols to paddy management

“…Thus, frequent inundation leads to reduced rates of OM decomposition (Sahrawat, 2004(Sahrawat, , 2005, which may result in increases in OM storage (Cheng et al, 2009;Kögel-Knabner et al, 2010). Most OM storage under paddy management is restricted to the puddled layer Chen et al, 2011;Kalbitz et al, 2013;Roth et al, 2011;Wissing et al, 2011). The paddy-specific dense plow pan prevents OM from entering deeper soil layers, likely due to less deep rooting and diminished vertical water fluxes Lennartz, 2006, 2007;Kalbitz et al, 2013).…”