Soil Nutrients and Aggregate Composition of Four Soils with Contrasting Textures in a Long-Term Experiment

Soil texture, coupled with environmental factors, plays an important role in soil nutrient conservation. In order to evaluate the effect of soil texture on soil fertility of fluvo-aquic soil, a long-term experiment that included four fluvo-aquic soils with different textures (sand, sandy loam, sandy clay loam and loamy clay) was established in 1990. In 2017, soil samples were collected from the 0–20 cm depth to measure soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP), total potassium (TK), alkali-hydrolysable nitrogen (AN), available phosphorus (AP) and available potassium (AK) concentrations, water-stable aggregate distribution, and aggregate-associated carbon and nitrogen. After 27 years of the experiment, the sand content decreased and the silt and clay content increased in sand and sandy loam, while the sand content increased and the silt content decreased in loamy clay. The SOC, total and available N, P and K (except TK) increased more in coarse-textured soils than in finely textured soils, while the concentrations of SOC, TN, TP, AN and AK increased significantly with a decrease in the sand content. The SOC, TN, AN and AK concentrations in loamy clay were 2 to 3 times higher than those in the sand treatment. The mass percentage of each aggregate fraction, especially the content of macroaggregates, increased significantly with a decrease in the sand content. Soil aggregates play an important role in SOC and TN storage. 75–84% of SOC and 67–81% of TN were stored in macroaggregates and microaggregates. It was concluded that soil particle composition was affected by long-term cultivation. The finely textured soils had higher SOC, TN and TP storage capacity and provided more AN and AK. The formation and stability of aggregate structures were limited by the high content of sand particles, which led to a negative influence on the storage and supply of SOC and N in the fluvo-aquic soils.