Planted forests intensified soil microbial metabolic nitrogen and phosphorus limitation on the Loess Plateau, China

Abstract

Appropriate plantation density is conducive to sustainable forest development. It is no consistent conclusion on how the construction of plantations, especially stand density, may affect soil enzymes activities and stoichiometry. The surface soil (0–20 cm) was sampled from Pinus tabuliformis plantations with low- (725–875 stem·hm−2), medium- (1375–1500 stem·hm−2), and high- (2150–2200 stem·hm−2) stand density and natural P. tabuliformis forests with medium- (1250–1400 stem·hm−2) and high- (2000–2700 stem·hm−2) stand density on the Loess Plateau in China. We measured the activities of C-, N-, and P-degrading enzymes (β-1,4-glucosidase, BG; β-1,4-N-acetylglucosaminidase, NAG; leucine aminopeptidase, LAP; and alkaline phosphatase, AP) and other major environment factors (i.e., vegetation characteristics, soil physicochemical properties, and microbial biomass). We found that (1) the activities of soil C- (BG), N- (NAG + LAP), and P- (AP) degrading enzymes were significantly higher in plantations than in natural forests owing to the lower soil nutrients content. (2) The soil enzyme stoichiometry ratio (C:N:P) of the natural forests and plantations deviated from the global average. Furthermore, the C:N and C:P ratios significantly decreased due to the significant increase of N- and P- degrading enzymes activities in medium- and high-density plantations. (3) Vector analysis showed that soil microbes were limited by C and N in natural forests. On the other hand, afforestation alleviated microbial C limitation, intensified N limitation, and caused P limitation. (4) The RDA analysis showed that soil enzymes activities and stoichiometry were mainly affected by soil organic C, available P, and microbial biomass C. Low-density P. tabuliformis plantations are conducive to improving ecological forest structure and plant diversity and may reduce soil microbial N and P limitation.