Microbial/biochemical indicators showing perceptible deterioration in the topsoil due to deforestation

Microbial parameters have been used to monitor changes in the topsoil under different land covers. Soils from five land cover systems common in northern Iran, including virgin natural forest, Alnus subcordata C.A.M. plantation, Quercus castaneifolia C. A. Mey plantation, Cupressus sempervirens var. horizontalis plantation and degraded natural forest were used to assess the effects of land cover changes on soil. We tested the following hypotheses: (i) deforestation would markedly influence topsoil C, N and P microbial ratios which make it distinct from the natural forest and plantations in the long-term, (ii) virgin natural forest and the establishment of plantations promote soil enzyme activities and the fractions of particulate organic matter (POC and PON) and dissolved organic matter (DOC and DON), 30 years after planting. Sixteen samples per land cover were taken from top 15 cm soil and characterized with respect to pH, organic C, total N, N mineralization, NH4+, NO3−, available P, fine root biomass, enzymes activities (urease, acid phosphomonoesterase, arylsulfatase and invertase), basal respiration (BR), substrate inducted respiration (SIR), microbial biomass-C, -N and -P (MBC, MBN and MBP), POC, PON, DOC and DON. There were no significant differences in BR/MBC, MBC/Corg, BR/SIR and MBC/MBN microbial indices among the plantations, virgin and degraded natural forests. The values of soil MBC/MBP were increased, more than one and half-fold, following deforestation (12.82) compared to virgin natural forest (8.53) site. The natural forest and establishment of plantations reduced the ratio of MBN/MBP, while the greater amount (1.14) was observed under degraded natural forest. With replacement in the type of tree species from Carpinus betulus and Parrotia persica in the natural forest to Quercus castaneifolia plantation, almost 30 years after planting, POC/PON increased significantly in the topsoil (13.29), about two-three fold, compared to virgin (7.13) and deforested (3.96) natural forests. The soil DOC/DON ratio varied significantly and it increased by about one and half-fold, following the growth of Quercus castaneifolia (2.87) and Cupressus sempervirens (2.68) trees compared to the other ecosystems. Based on PCA, virgin natural forest and Alnus subcordata plantation presented a good condition of soil fertility, more microbial and enzyme activities with a low ratio of microbial indicators. In general, microbial/biochemical indicators showed perceptible deterioration in the topsoil due to deforestation. As a conclusion, soil enzyme activity, MBC/MBP, MBN/MBP, POC/PON and DOC/DON are robust and sensitive indicators that can be used to measure long-term variations in the topsoil due to changed land cover.