Soil physical quality as influenced by long-term fertilizer management under an intensive cropping system.

In China's major rice (Oryza sativa L.) production regions, the traditional fertilization modes are challenged by the continued decrease in manure and increase in mineral fertilizer. However, limited information exists on the influences of long-term fertilizer management on soil organic carbon (SOC) and soil physical properties under the intensive rice production system in southern China. The objective of this study was to characterize the changes of soil physical properties as related to mineral fertilizer, crop residues, and manure application based on a long-term field experiment. The experiment, initiated in 1986, has five treatments: unfertilized, mineral fertilizer alone, rice residues plus mineral fertilizer, low manure rate plus mineral fertilizer, and high manure rate plus mineral fertilizer. The cropping system consists of barley (Hordaum vulgare L.), early rice, and late rice, three crops in a year. In May 2006, after barley harvest, soil samples were collected from the 0∼10 cm and 10∼20 cm layers to determine SOC concentration, aggregate size distribution, bulk density (Pb), saturated hydraulic conductivity (K,), and soil water characteristic curves (SWCC). The results indicated that manure significantly reduced Pb, increased SOC concentration, soil aggregation, K~, transmission and storage porosity, as well as water retention capacity. Combined application of crop residue and mineral fertilizer also improved soil physical properties, but the improvement by mineral fertilizer alone was limited. Correlation analysis demonstrated that S, the slope of the SWCC at its inflection point, was closely associated with the selected physical parameters, suggesting S was an effective parameter for soil physical quality evaluation; Nevertheless, in applying the S-theory, a unified approach to define the residual water content should be considered. [ABSTRACT FROM AUTHOR]