Increase of soil nutrients and enzymatic activity by adding nitrogen and phosphorus to Larix principis-rupprechtii plantation

【Objective】 A field experiment was conducted to study effects of exogenous nitrogen and phosphorus fertilization on soil nutrients and enzyme activities, and select proper fertilizer amounts for the half-mature Larix principis-rupprechtii plantation in the Qinling Mountains of China. 【Methods】 Nitrogen and phosphate fertilizers were applied individually or together in three levels in a 20-year-old half-mature Larix principis-rupprechtii plantation in mid April 2012. Soil samples in 0-20 cm depth were collected from May to October 2012, and the soil nutrient contents and enzyme activities were measured. The principal components analysis was used to select the optimum fertilization scheme. 【Results】 1) The application of nitrogen and phosphorus fertilization significantly increases the soil available nitrogen(N) and available phosphorus(P) contents, but decreases soil available potassium(K) content and pH. The increase levels of different treatments in soil available N was in the order of N2PN2N1PN1P, and available P in N1PPN2PN2N1, while the soil available K decrease was in the order of N2PN1PN2PN1 and pH in order of N2PN2PN1PN1. 2) The activities of soil invertase, phosphatase, and urease could be significantly increased, but that of hydrogen peroxidase decreased by the N and P fertilization. The increase levels of the soil enzyme activities for invertase are in the order of N1PN2PN1PN2, for phosphatase are N2PN1N1PN2P, and for urease are N2PN1PN1N2P, while the decreased of hydrogen peroxidase are in the order of N2PN2N1PPN1. 3) The responses of soil nutrient contents are sensitive to seasons. The available N, P and K contents decrease firstly and then increase during the year, with the minimum values appeared in plants in July and August. The changes of soil enzyme activities are complex, most often the maximum values are also appeared in July and August. 4) Nitrogen and phosphorus fertilizer alone, or their combined application could increase the contents of soil available N and available P. The soil available N is increased from 21.56 mg/kg to 35.77 mg/kg and soil available P from 1.86 mg/kg to 3.73 mg/kg. The soil fertility levels are improved by fertilization, with satisfactory effects with the N1P and N2P treatments. 5) Both the nitrogen and phosphorus fertilization could increase the activities of soil invertase, urease and phosphatase, but decrease hydrogen peroxidase activity slightly. The fertilization generally increases the soil enzyme activities. 6) The principal components analysis indicates that the fertilization could significantly improve soil fertility levels with the order of N1P N2P P N1 N2CK. The highest increase is 1.505 in N1P, then is 1.405 in N2P.【Conclusions】 Fertilization is capable of enhancing soil nutrients and enzyme activities, thus improving soil fertility and alleviating the decline in soil fertility caused by nitrogen and phosphorus deficiencies in the Larix principis-rupprechtii plantation. The proper fertilization amount is N 75 kg/hm2 and P2O575 kg/hm2 (N1P), followed by N 150 kg/hm2 and P2O575 kg/hm2 (N2P)