Phosphorous (P) is an essential nutrient for plant growth. However, excessive P in soils is often considered to be detrimental to surface water. Moreover, soil erosion and P losses are closely related to the stability of soil aggregates and to the distribution of nutrient elements within such aggregates. Red soils are the major land resource in subtropical and tropical areas and are characterized by low P availability.

In a paper in Environment Science and Pollution Research, researchers in the Institute of Subtropical Agriculture, Chinese Academy of Sciences (ISA) collected two pairs of subtropical red soil samples from a paddy field and an adjacent uncultivated upland to assess the availability of P for plants and the potential stability of P in soil by analysis of total P and Olsen P, and sequential extraction for the inorganic and organic P fractions in different aggregate size classes.

The team found soils under paddy cultivation displayed improved soil aggregate structure; altered distribution patterns of P fractions in different aggregate size classes, and to some extent had enhanced labile P pools. "Paddy cultivation significantly decreased the proportion of <2 mm size classes (p < 0.05), and increased the proportion of >2-mm size classes (p < 0.05)," said LI Baozhen, an associate professor in ISA.

"Comparison of those in the paddy soil with in the upland soil, the total P and Olsen-P contents were 50%-150% and 50%-300% higher, respectively. Higher inorganic and organic P fractions tended to be enriched in both the smallest and largest aggregate size classes compared to the middle size class (0.02-0.2 mm)."

Moreover, compared to upland soils, paddy soil had lower clay content and a markedly higher proportion of large aggregates (>2-mm aggregate fraction), suggesting that rice cultivation might actually increase the stability of aggregates and reduce the risk of soil surface runoff and erosion.

This study was supported financially by the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB15020401), the National Natural Science Foundation of China (41271483), Youth Innovation Promotion Association, CAS (2012273) and the Recruitment Program of High-end Foreign Experts of the State Administration of Foreign Experts Affairs awarded to Prof. Georg Guggenberger and Prof. Kazuyuki Inubushi (GDT20154300073), International cooperation and regional science and technology of Hunan Province (2015WK3044), and the State Scholarship Fund of China Scholarship Council.

The study entitled "Phosphorus Content as a Function of Soil Aggregate Size and Paddy Cultivation in Highly Weathered Soils" has been published in the September issue of Environment Science and Pollution Research, details could be found at http://link.springer.com/article/10.1007/s11356-015-5977-2.