Abstract
Mineral fertilizers and organic amendment can affect the various soil organic matter (SOM) pools and the distribution of organic carbon (OC) and nitrogen (N) in these pools. It is unknown how OC and N are distributed in different SOM pools under different long-term fertilization regimes. Therefore, this study aimed to examine the effects on OC and N concentrations in various SOM pools after 33 years of application of chemical fertilizer and organic amendment in Anhui Province in the Huang-Huai-Hai Plain, eastern China. This long-term experiment consisted of five fertilization treatments measuring changes in the OC and N concentrations in the soils and different SOM fractions of each experiment plot. Organic amendment increased the OC and N concentrations in the mineral-associated fraction, the coarse mineral-associated fraction and the aggregates compared with the values obtained without fertilizer application. Mineral fertilizer application alone increased the abovementioned indexes, but this increase was small. There was a small but significant increase in the OC and N concentrations in the free particulate fraction, and the change in magnitude had no obvious effect on the total OC (TOC) and total N (TN) concentrations in soils. More than 80% of the water-stable aggregate-associated C was stored in macroaggregates >2 mm in size. More than 60% of the TOC and TN accumulated within mineral associations in the soil, and organic amendment increased this proportion to 80%. These results suggest that the OC in Vertisols is dominated by mineral-associated OC and that the effect of organic amendment on mineral-associated OC is obvious.
Highlights
Soil organic matter (SOM) plays an important role in agroecosystem services [1] and crop yields [2] by maintaining and supplying nutrients to crops, forming soil aggregates and enhancing the water storage capacity
More than 60% of the total OC (TOC) and total N (TN) accumulated within mineral associations in the soil, and organic amendment increased this proportion to 80%
These results suggest that the organic carbon (OC) in Vertisols is dominated by mineral-associated OC and that the effect of organic amendment on mineral-associated OC is obvious
Summary
Soil organic matter (SOM) plays an important role in agroecosystem services [1] and crop yields [2] by maintaining and supplying nutrients to crops, forming soil aggregates and enhancing the water storage capacity. According to Marriott and Wander [8], the bulk SOC pool can be separated into conceptual aggregation fractions using a fractionation procedure based on different protection mechanisms These separated fractions are called free particulate fraction (unprotected SOC inter-aggregate), occluded-microaggregate particulate fraction (or physically protected SOC) and mineral-associated fraction (chemically and biochemically protected SOC). The macro and light OM fractions are decomposed into smaller and finer OM fractions, which form the stabilized fraction with physical and/or chemical protection called the mineral-associated OM (MOM) and notably contribute to the total OC (TOC) [11]. This fraction is usually a recalcitrant OM pool due to it being protected with a slower turnover rate [7]
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