Effects Of N And P Addition On Soil Carbon And Nitrogen Dynamics In A Subtropical Evergreen Broad-leaved Forest

The seasonal patterns of soil active organic carbon, nitrogen and nitrogen mineralization and their responses to N and P addition were investigated in a subtropical evergreen broad-leaved forest at Zhawan Natural Reserve, in southern Anhui. The results showed that the concentrations of NH4+-N, NO3--N and available N in 0-30 cm soil layer were, respectively,9.42,1.66 and 11.08 mg kg-1. On the mid-slope, NH4+-N concentrations in 0-20 cm soil layer for the different treatments followed the order of N addition> N+P addition> CK; and on the flat ridge site, it also followed the order of N+P addition> N addition> CK. On the mid-slope, NO3--N concentrations in 0-10 cm soil layer for the different treatments followed the order of N+P addition> N addition> CK, while in 10-20 cm,20-30 cm soil layer were all followed the order of N addition> N+P addition> CK. The treatments of N addition and N+P addition were significantly (p<0.05) increased soil NO3--N concentrations. On the flat ridge, NO3--N concentrations in 0-10 cm,10-20 cm,20-30 cm soil layer for the different treatments were all followed the order of N addition> N+P addition> CK. The concentrations of NH4+-N in 0-10 cm,10-20 cm,20-30 cm soil layer were 12.59,8.51,7.17 mg kg-1, respectively. While NO3--N concentrations in 0-10 cm,10-20 cm,20-30 cm soil layer were, respectively,2.48,1.24,1.09 mg kg-1. Both NH4+-N and NO3--N concentrations decreased with soil depth for the top 30-cm layer.The concentrations of DOC in 0-30 cm soil layer were 242.53 mg kg-1. On the mid-slope, the DOC concentrations differed amongst the different treatments with the order of N addition> N+P addition> CK. Significant difference was found between N addition and N+P addition, CK (p<0.05). On the flat ridge, DOC concentrations of the different treatments in 0-10 cm soil layer were, respectively,315.86,275.79,274.1 mg kg-1, No significant difference appeared between N addition and N+P addition (p> 0.05). On the mid-slope, DON concentrations in 0-10 cm soil layer for the different treatments followed the order of N addition (39.9 mg kg-1)> N+P addition (33.99 mg kg-1)> CK (31.21 mg kg-1),10-20 cm,20-30 cm soil layer response to nitrogen and phosphorus addition with the same of 0-10 cm soil layer. The DON concentrations of the different treatments in 0-10 cm soil layer were very closed to each other on the flat ridge, their concentrations were 37.95,37.72,37.12 mg kg-1, respectively. The DOC concentrations ranged from 126 to 298 mg kg-1 for the 0-30 cm soil layer.The concentrations of DOC and DON changed with the seasons followed the trend of "lower-increasing-lower-increasing". The concentrations of DOC in 0-10 cm, 10-20 cm,20-30 cm soil layer were, respectively,277.22,244.34,206.02 mg kg-1; While DON concentrations in 0-10 cm,10-20 cm,20-30 cm soil layer were 36.81、 27.68、23.53 mg kg-1.The mean concentrations of MBC in 0-30 cm soil layer were 510.14 mg kg-1, with 520.06 mg kg-1 on the mid-slope and 500.22 mg kg-1 on the flat ridge. On the mid-slope, the concentrations of soil MBC were lower in the N and N+P additions than CK in January, May and July. This suggests that N and N+P additions can decrease soil MBC. On the flat ridge, MBC concentrations for the different treatments followed the order of N+P addition> CK> N addition, indicating that N addition can decrease soil MBC while N+P addition increases soil MBC. MBC concentrations ranged from 209 to 767 mg kg-1 for the 0-30 cm soil layer on the flat ridge, while from 236 to 742 mg kg-1 on the mid-slope. The maximum concentration of soil MBC appeared in November of 2012 and march of 2013. On the mid-slope, the MBN concentrations differed amongst the different treatments with the order of N addition (144.57 mg kg-1)> N+P addition (130.9 mg kg-1)> Ck (130.5 mg kg-1), while was N+P addition (155.31 mg kg-1)> CK (141.26 mg kg-1)> N addition (127.11 mg kg-1) on the flat ridge. The concentrations of MBN changed with the seasons followed the trend of "lower-increasing-lower-increasing".On the mid-slope, the soil net ammonification rate in 0-10 cm soil layer for the different treatments followed the order of CK (236.46 μg·kg-1·d-1)> N+P addition (162.23 μg·kg-1·d-1)> N addition (154.19 μg·kg-1·d-1), While 10-20 cm followed the order of CK (51.27 μg·kg-1·d-1)> N+P addition (49.18 μg·kg-1·d-1)> N addition (18.44 μg·kg-1·d-1). Both the mid-slope and the flat ridge soil net nitration rate in 0-10 cm soil layer for the different treatments followed the order of N+P addition> N addition> CK.