| Tea is a globally important crop and is unusual because it both requires and acidifies the soil in which it grows. In spite of the low pH, high NO3-accumulation in and N2O emission from tea soils have been found, resulting in low N use efficiency and a great potential for diffuse pollution. In this study, represent tea soils with different basic properties, from tea gardens with varied management levels and planting models were collected from Zhejiang province, eastern China. A special attention was focused on tea soils with different production levels and tea stand ages, and their adjacent forest or/and vegetable soils in which tea soil originated. A series of field investigation and laboratory experiments were carried out to study soil microbial properties, biomass C, N and P, microbial quotient, net and gross nitrification, microbial nitrifiers, denitrification and N2O emission rate and their impact factors. The main results are as follows:1. The microbial biomass C from75tea soils ranged from38.1to607.2mg/kg with an average of208.1mg/kg, microbial quotient C (ratio of microbial biomass C to total organic C) ranged from0.21%to4.55%with a mean of1.25%; the microbial biomass ninhydrin-N ranged from1.76to76.14mg/kg with an average of20.67mg/kg, microbial quotient ninhydrin-N ranged from0.26%to9.09%with a mean of1.80%; the microbial biomass P ranged from undetected to60.07mg/kg with a mean of14.93mg/kg, microbial quotient P ranged from0to10.20%with a mean of3.06%. Compared to other arable, grass and forest soils in literature, the microbial biomass, microbial quotient C and ninhydrin-N in tea soils were lower. However, the microbial biomass P was higher.2. The microbial biomass content in tea garden soils was strongly influenced by tea cultivation intensity (production level), tea planting duration, soil pH, fertilizer application and tea planting model. Soil pH was highest in the forest soil and decreased with increasing productivity and age of tea stand. Soil total organic C, N, P and available nutrients significantly increased. Microbial biomass C, ninhydrin-N, PLFAs declined in the order forest> low> high> middle production, and biomass C and ATP declined in the order tea stand age50> age9> forest> age90. Microbial biomass ninhydrin-N and microbial quotient N declined in the order forest≥age9> age50> age90. Soil pH had a strong influence on the microbial biomass, demonstrated by positive linear correlations with: microbial biomass C, microbial biomass ninhydrin-N, PLFAs, the microbial quotient C, ninhydrin-N and P, the basal respiration rate and specific respiration rate. Appropriate fertilization could increase microbial biomass P. However, over application of N fertilizers reduced microbial biomass C, microbial biomass ninhydrin-N. With the increase of N application, the ratio of biomass C to total organic C, biomass ninhydrin-N to total N and biomass P to total P all significantly decreased. A principal component (PC) analysis of PLFA data showed consistent shift in the community composition with productivity level and stand age. The Gran positive bacterial PLFA biomarkers were significantly higher than Gran negative bacterial and fungal PLFA biomarkers. The ratio of fungal:bacterial PLFA biomarkers was negatively and linearly correlated with specific respiration rate in the soils (R2=0.93, p<0.05). Converting tea fields from conventional to organic increased soil pH and total organic C, the longer under organic cultivation, the higher pH and total organic C. Organic tea fields had higher microbial biomass C, ninhydrin-N, P and their microbial quotients than conventional fields.3. In spite of low pH, tea garden soils had strong nitrification. The NO3--N concentrations in130tea field soils ranged from0to286.8mg/kg with an average of41.7mg/kg. About41%and12%of soil samples below20and above100mg/kg, respectively. Soil net nitrification rate ranged from-6.08to6.54mg/kg-d with an average of1.62mg/kg-d. About10%soils were in N immobilization. The NO3--N and net nitrification rate increased with increase of N application rate. The majority of soils with very higher NO3-accumulation had higher N application rate, pH below4.0, and even negative net nitrification rate. The gross nitrification rate from15soils ranged from0.75to11.56mg/kg.d with a mean of4.66mg/kg.d. Soil microbial NO3--N consumption rate ranged from0.29to7.67mg/kg.d with a mean of3.01mg/kg.d, accounting for33.7%-76.7%(mean61.5%) of gross nitrification rate. Nitrificaton rate in the tea soils had no significantly different with the neutral pH vegetable soil, but significant higher than the forest soil.4. Type of land use and its management, tea planting model and duration were more effective to impact soil nitrification rate than soil pH. The net and gross nitrification rate was highest in vegetable soil, followed by tea soils, and the lowest in the forest soil. Net nitrification rate decreased in the order forest> age50> age90> age9tea stand, and organic> conventional> abandoned soils. Gross nitrification rate declined in the order tea age50> age90> age9> forest. The nitrification rate increased with increasing tea production and N application rates. The NH4+added to the vegetable soil was immediately and completely nitrified, resulted in nitrification pattern in first-order kinetics, but that in the tea and forest soils was nitrified more slowly with nearly linear nitrification pattern. About60%-80%of the added NH4+in the tea and forest soils was nitrified during35days of incubation, indicating nitrifiers do exist in these soils. High nitrogen application rate is the main cause of NO3-accumulation in tea soils and should be reduced to increase N use efficiency and minimize NO3-pollution of water resources. Liming did not increase soil gross nitrification rate.5. The key microorganism for strong nitrification in tea soils were ammonia-oxidizing archaea (AOA), instead of ammonia-oxidizing bacteria (AOB). No AOB were found by traditional incubation method in the soil of high tea production field. Soil DNA sequence analysis after quantitative polymerase chain reaction (Q-PCR) and terminal-restriction fragment length polymorphism (T-RFLP) showed the abundance of AOA and AOB, and nitrification potentials were significantly higher in the tea soils than adjacent forest soils. The AOA amoA gene copy numbers ranged from5.5×104to2.4×107/g with an average of1.36X107/g, and the AOB amoA gene copy numbers ranged from below detection limits to4.40×106/g with a mean of1.20×106/g. A significant relationship between AOA abundance and nitrification potential (p<0.001) was found, but not between the AOB abundance and nitrification potential. Soil pH had a significantly negative relationship with AOB abundance, but not with AOA abundance. There was an exponential increase in the ratio of AOA to AOB amoA gene copies with decreasing soil pH values in the tea gardens. The AOB copy numbers were below detection limits in some highly acidic tea soils, but these soils still had high nitrification rates. These results suggest that nitrification is mainly driven by AOA and not AOB in highly acidic tea soils. We also found that different genotypes of AOA and AOB adapt to a particular soil pH and N content which suggest that functional microbial populations also follow niche based community assembly.6. High N2O emission rate was found in tea garden soils, high N application rate was the main cause. The N2O emission rate in142tea soils ranged from0to4960μg/m2.h with an average of467μg/m2.h. It decreased in the order high> middle> low tea production and vegetable> forest soils. Its annual change was in accordance with the soil temperature. N2O emission rate was significantly increased with the increase of N application rate and of soil moisture contents, which had a synergistic effect on N2O emission rate. The annual fertilizer-induced emission factor ranged from1.43%to3.44%with a mean of2.28%. High soil organic C increased N2O emission, but no effect of soil pH on N2O emission was found. There were a significant reduction of N2O emission by bactericides and significant relationship between N2O emission rate and nitrate reductase, indicating that N2O emitted by microbial denitrification. |
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