Fate of 14C-labeled dissolved organic matter in paddy and upland soils in responding to moisture

Soil organic matter (SOM) content in paddy soils is higher than that in upland soils in tropical and subtropical China. The dissolved organic matter (DOM) concentration, however, is lower in paddy soils. We hypothesize that soil moisture strongly controls the fate of DOM, and thereby leads to differences between the two agricultural soils under contrasting management regimens. A 100-day incubation experiment was conducted to trace the fate and biodegradability of DOM in paddy and upland soils under three moisture levels: 45%, 75%, and 105% of the water holding capacity (WHC). 14C labeled DOM, extracted from the 14C labeled rice plant material, was incubated in paddy and upland soils, and the mineralization to 14CO2 and incorporation into microbial biomass were analyzed. Labile and refractory components of the initial 14C labeled DOM and their respective half-lives were calculated by a double exponential model. During incubation, the mineralization of the initial 14C labeled DOM in the paddy soils was more affected by moisture than in the upland soils. The amount of 14C incorporated into the microbial biomass (2.4–11.0% of the initial DOM-14C activity) was less affected by moisture in the paddy soils than in the upland soils. At any of the moisture levels, 1) the mineralization of DOM to 14CO2 within 100days was 1.2–2.1-fold higher in the paddy soils (41.9–60.0% of the initial DOM-14C activity) than in the upland soils (28.7–35.7%), 2) 14C activity remaining in solution was significantly lower in the paddy soils than in the upland soils, and 3) 14C activity remaining in the same agricultural soil solution was not significantly different among the three moisture levels after 20days. Therefore, moisture strongly controls DOM fate, but moisture was not the key factor in determining the lower DOM in the paddy soils than in the upland soils. The UV absorbance of DOM at 280nm indicates less aromaticity of DOM from the paddy soils than from the upland soils. At any of the moisture levels, much more labile DOM was found in paddy soils (34.3–49.2% of the initial 14C labeled DOM) compared with that in upland soils (19.4–23.9%). This demonstrates that the lower DOM content in the paddy soil compared with that in the upland soil is probably determined by the less complex components and structure of the DOM.