| Nitrogen and phosphorus mineralization in soil is a key and direct source of soil nitrogen and phosphorus nutrients, and affected by various biotic and abiotic factors resulted from the changes in climate zones, altitudes, landforms and other natural conditions. As yet, little attentions have been given to the changes in soil nitrogen and phosphorus mineration with the elevated altitudes at different critical periods. Minjiang River is situated in the upper reaches of Yangtze River and eastern Tibet Plateau, of which the vegetations and soils vary greatly with the elevated altitudes, and provide with a natural lab for studying the changes of soil nitrogen and phosphorus mineralization with the elevated altitudes at different critical periods. In order to understand the dynamics of soil nitrogen and phosphorus mineralization along the levated elevatelevation gradients, an in situ soil core incubation experiments were therefore conducted in the mid-subtropical evergreen broad-leaved forest (E1,442 m. a.s.l.), northern subtropical evergreen broad-leaved forest (E2,1050 m. a.s.l.), dry valley (E3,1563 m. a.s.l.), ecotone between dry valley and montane forest (E4,2158 m. a.s.l.), subalpine dark coniferous forest (E5, 3028 m. a.s.l.), alpine coniferous forest (E6,3593 m. a.s.l.) and alpine meadow (E7,3994 m. a.s.l.). Consequently, the rates of ammonification, nitrification, denitrification, and net nitrogen and phosphorus mineralization in both soil organic layer and mineral soil layer were determined at different critical periods in the growing seasons and non-growing seasons from September 2013 to December 2015. Meanwhile, the temporal change of nitrogen and phosphorus input, output and mineralization process in different ecosystems were simultaneously monitored so as to understand the sources and sinks of nitrogen and phosphorus in soils.After two years monitoring, the nitrogen input all showed the tendency of significant decreasing with the rising of elevation in the ecosystems from E1 (85.32 kg/hm~2) to E3 (19.43 kg/hm~2). On the contrary, that increased with the rising of elevation in the ecosystem from E4 to E7, but it had not significant difference (p>0.05). The input of total nitrogen composed of nitrate, organic nitrogen, ammonia and nitrite, nitrate concentration ranged from 46% to 48% in input to all 4 kinds of nitrogen components in E1, E2, E5 and E6. Ammonia concentration ratio at E6 (10%) and E7 (9%) showed significant higher than that in other ecosystems. The input of total phosphorus in E, [6.57 kg/(hm~2·a)], E2 [8.02 kg/(hm~2·a)], E6 [4.66 kg/(hm~2·a)] and E7 [3.40 kg/(hm~2·a)] showed significant higher than that in other ecosystems [average in 2.10 kg/(hm~2·a)], but only 0.1%-0.2% of total P input is dissolved reactive phosphorus (DRP). In addition, more than 50% of nitrogen and 85% of phosphorus input were concentrated in onset of freezing stage and early growing season, but more than 82% of nitrate input in thawing stage. The change tendency of phosphorus input was similar to nitrogen input. There was positive correlation between the total nitrogen, nitrate, ammonia, dissolve reactive phosphorus and dissolve total phosphorus with elevation.Nitrogen leaching from soil organic layer in the ecosystems along elevation E4 to E7 showed positive correlation with elevation. From E1 [61.50 kg/(hm~2-a)] to E3 [23.26 kg/(hm~2-a)] and from E5 [39.02 kg/(hm~2-a)] to E7 [14.05 kg/(hm~2-a)], the nitrogen leaching from mineral soil layer showed decreasing tendency with elevation, but from E3 [23.26 kg/(hm~2·a)] to E5 [39.02 kg/(hm~2-a)], nitrogen leaching showed positive correlation with elevation. Compare with the nitrate input, the leaching of nitrate concentration from mineral soil layer significant reduce to 2%-4%in each sites, but leaching of ammonia from mineral soil layer increased significantly, and ammonia leaching in the site of E1 and E2 was 50% and 53%, respectively. The results show that change tendency of soil phosphorus leaching with elevation gradient is similar to soil nitrogen leaching. From E1 to E7, phosphorus leaching of each site was 4.70 kg/(hm~2·a),1.60 kg/(hm~2·a),2.43 kg/(hm~2·a),1.10 kg/(hm~2·a),2.16 kg/(hm~2·a),1.17 kg/(hm~2·a) and 0.31 kg/(hm~2·a). Moreover, the ratio of dissolve reactive phosphorus leaching was 15% in E7. The soil nitrogen and phosphorus leaching was concentrated in thawing stage. In the other periods, the leaching of nitrogen components in late growing season was significant higher than early growing season. Soil nitrogen and phosphorus leaching was showed decreasing tendency from thawing stage to next deep freezing period.Over the course of two years monitoring, soil total nitrogen, total phosphorus and inorganic phosphorus increased significantly in the second year, but soil ammonia, nitrate and nitrite concentration decreased significantly in the second year. In Minjiang river basin, the soil organic layer and mineral soil layer of E4 had the lowest total nitrogen, nitrate, and ammonia concentration. From E1 to E3, soil total phosphorus, inorganic phosphorus, nitrite concentration in soil organic layer and mineral soil layer and nitrate and ammonia concentration in mineral soil layer was positive correlation with elevation. Moreover, the different between sites was smaller. In different key periods, the nitrogen and phosphorus concentration had significant differences, and the soil organic layer and mineral soil layer had the highest concentration of total nitrogen, total phosphorus and inorganic phosphorus in onset of freezing stage, the soil nitrate, ammonia and nitrite concentration peak was in thawing stage, late growing season and deep freezing period, respectively. In addition, the concentration of total nitrogen, total phosphorus and inorganic phosphorus between five key periods were not significantly differentia (p>0.05).The soil organic layer net nitrogen mineralization rates increased from-1.83 g/(m~2·a) (E1) to 4.18 g/(m~2·a) (E5), and that decreased significantly from E5 to .012 g/(m~2·a) (E7) with elevation in Minjiang River basin. The mineral soil layer of E2 [1.60 g/(m~2·a)] and E4 [-0.31 g/(m~2·a)] had the highest and the least nitrogen mineralization rates, respectively. The soil nitrification, ammonification and denitrification rates in E2 and E5 showed significantly higher than other sites (p<0.05), and the value of cumulative nitrification and ammonification rates in E2 mineral soil layer were 1.13 g/(m~2·a) and 0.01 g/(m~2·a), respectively. There had no significant difference on denitrification and phosphorus mineralization rates in soil organic layer, and there had the highest denitrification [-0.02 g/(m~2·a)] and phosphorus mineralization rates [0.02 g/(m~2·a)] in E5 site. Interestingly, the mineral soil layer of E3 site had the lowest nitrification [-3.11 g/(m~2·a)], ammonification [-1.71 g/(m~2·a)] and denitrification [-0.43 g/(m~2·a)] rates than other sites, but phosphorus mineralization rates [0.05 g/(m~2·a)] showed 3 to 10 times higher than other sites. In addition, the soil nitrogen mineralization processes were concentrated on onset of freezing stage and early growing season, and soil nitrification and ammonification rates showed most active in thawing stage and late growing season, respectively. However, soil phosphorus mineralization processes were concentrated on onset of freezing stage and early growing season.In summary, the organic soil layer and mineral soil layer had highly significant differences in soil nitrogen and phosphorus mineralization along elevation gradient in Minjiang River basin. And the soil organic layer of E5 and mineral soil layer of E2 had the highest nitrogen mineralization rates, respectively. Compare with E5, the nitrification, ammonification and denitrification rates showed significant increasing in soil organic layer of E6 site. Mineral soil layer of E3 had the lowest nitrogen mineralization rates, but had the highest phosphorus mineralization rates. Soil pH and nitrogen conponents concentration was positive correlation with soil nitrogen mineralization, and highly C/N value significant contribution to nitrogen minerzlization processes (p<0.01). Therefore, soil nitrogen mineralization processes was mainly controlled by substrate concentration. There were significant differences in soil nitrogen and phosphorus mineralization among key periods, but the mineralization processes of different nitrogen and phosphorus components were corresponding to different periods of incubation. Specifically, the nitrification, denitrification, ammonification and phosphorus mineralization primarily occurred in onset of freezing stage, thawing stage and early growing season. |
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