Reforestation and slope-position effects on nitrogen, phosphorus pools, and carbon stability of various soil aggregates in a red soil hilly land of subtropical China

The impact of slope position on forest restoration processes may dominate the interaction among topsoil aggregates, nutrients, and organic carbon (C) stability following reforestation on degraded hilly land. Six paired plots of shrubland and Masson pine (Pinus massoniana Lamb.) plantation reforested on shrubland were selected at different positions along a 25 degrees slope in subtropical China. Soil and ecosystem nitrogen (N), phosphorus (P), and organic C stocks were quantified using a combination of excavation, harvest, and modeling methods. Carbon stability in differently sized soil aggregates was measured using dry sieving and closed-jar alkali-absorption methods. Reforestation increased total ecosystem C, N, and P stocks, but did not alter soil C and P stocks, and decreased N stocks at 0-75 cm soil depth by 18%. The shift from shrubland to forests increased organic C stability for various soil aggregates, particularly with a mean increase of 34% and 30% at lower and middle slope positions, respectively. The slope-position effect was largely attributable to the increased mean mass diameter of soil aggregates and the reduced organic C mineralization potential in larger soil aggregates relative to smaller soil aggregates. Soil aggregates had more stable organic C in forests than in shrubland due to increase in the carbon to nitrogen (C/N) and carbon to phosphorus (C/P) ratios with reforestation. Slope position influenced restoration effectiveness in the red soil hilly region.