| Soil nitrogen supplying capacity is the important parameter in determining the amount of nitrogen fertilizer application. Nitrate that existing significant variation in different dry land calcareous soils, was the main form of soil initial mineral nitrogen, and ammonium was low and stable in dry land soils. Therefore, when we use crop nitrogen uptake to estimate soil potential nitrogen supplying capacity (mineralized nitrogen during the period of plant growing), nitrate content may influence the reliability of the estimation. So pot experiment with wheat and maize as test crops was carried out which including leaching and without leaching soil initial nitrate. We compared the 9 biological and 12 chemical methods which reflecting soil nitrogen supplying capacity. Test soils were taken from 25 different types of calcareous surface soils on Loess Plateau. The 9 biological methods include waterlogged incubation, aerobic incubation for 2 weeks and for 4 weeks, dry-wet alternation aerobic incubation for 2 weeks, long-term alternate leaching aerobic incubation, short-term leaching aerobic incubation, nitrogen mineralization potential (N0), microbial biomass carbon (BC) and microbial biomass nitrogen (BN) method. The 12 chemical methods include soil total nitrogen (TN), soil organic matter (OM), mineral nitrogen (Nmin), alkaline KMnO4 method, acid KMnO4 method, H2SO4-KMnO4 method, phosphate-borate buffer method, boiling KCl method, hot KCl water bath method, acid KCl method, boiling water method and NaHCO3 UV method. Among these methods, the dry-wet alternation aerobic incubation and aerobic incubation for 4 weeks were the modification of the aerobic incubation for 2 weeks method according to the actual farmland moisture. And the H2SO4-KMnO4 method was a modification of the acid KMnO4 method. We also studied the correlations between soil microbial biomass carbon (BC), soil microbial biomass nitrogen (BN) and soil particle composition, nitrogen mineralization potential (N0), and analyzed the differences of soil microbial biomass carbon (BC), soil microbial biomass nitrogen (BN) and nitrogen mineralization potential (N0) in different calcareous surface soils on Loess Plateau. The results as follows: 1. The results of these biological methods showed that the nitrogen mineralization potential (N0) was significantly correlated with crop uptake nitrogen with leaching soil initial nitrate (r=0.790, p<0.01), and the correlation coefficient was highest as an index of soil nitrogen supplying. The values of mineralized nitrogen obtained by aerobic incubation for 4weeks, dry-wet alternation aerobic incubation method and long-term alternate leaching aerobic incubation method had remarkably positive correlations with crop uptake nitrogen with leaching soil initial nitrate (Correlation coefficient were 0.777, 0.768 and 0.764, respectively, p<0.01). The correlation coefficient was lower between the water logged incubation method and crop uptake nitrogen with leaching soil initial nitrate (r=0.530, p<0.01). The correlation coefficients between those biological methods and crop uptake nitrogen increased significantly while including the soil initial nitrate. The correlation coefficient were highest for short-term leaching aerobic incubation method and the aerobic incubation for 2 weeks method with crop uptake nitrogen (r=0.963,p<0.01) which including the soil initial nitrate. The nitrogen obtained by aerobic incubation for 4weeks, dry-wet alternation aerobic incubation for 2 weeks method, long-term alternate leaching aerobic incubation method and N0 had remarkably positive correlations with crop uptake nitrogen without leaching soil initial nitrate (Correlation coefficient were 0.962, 0.959, 0.825 and 0.812, respectively, p<0.01). It could be seen that high contents of soil initial mineral nitrogen, especially nitrate affected the correlations between the biological methods and crop uptake nitrogen. The correlation coefficient between the microbial biomass carbon (BC), microbial biomass nitrogen (BN) with crop uptake nitrogen with leaching initial nitrate were respectively 0.555 (p<0.01) and 0.465 (p<0.05). The correlation between BC, BN with crop uptake nitrogen without leaching initial nitrate were not significantly. In conclusion, due to long-term alternate leaching aerobic incubation method and N0 needed long term incubating, they were not suitable for general analysis in laboratory. Thus, dry-wet alternation aerobic incubation for 2 weeks was chosen as better index for evaluating calcareous soil N supply due to some methods having disadvantages and not suitable the actual farmland character.2. The results of these chemical methods indicated that soil organic matter (OM) and total nitrogen (TN) can stable reflect soil nitrogen supplying capacity in some degree. However, the sensitivity of these two methods was low. The initial mineral nitrogen in those calcareous soil, especially soil nitrate, had significant correlation with crop uptake nitrogen without leaching soil initial nitrate (r=0.884, p<0.01), but the correlation was not significant for crop uptake nitrogen with leaching soil initial nitrate (r=0.472, p<0.05). The results indicated that initial mineral nitrogen could reflect soil nitrogen supplying capacity at present, but can't reflect soil potential nitrogen supplying capacity. The high content of soil initial nitrate affected the correlations between the chemical methods and crop uptake nitrogen. The phosphate-borate buffer method was a good soil nitrogen supplying index for reflecting soil total nitrogen supplying capacity (reflecting both soil present and potential nitrogen supplying capacity), but not well reflecting soil potential nitrogen supplying capacity. The acid KMnO4 method reflected both soil potential and total nitrogen supplying capacity. The amount of ammonium released by H2SO4-KMnO4 method was similar to that extracted by boiling KCl method and hot KCl water bath method. The H2SO4-KMnO4 method was equal to acid KMnO4 method for reflecting soil total nitrogen supplying capacity, but it was inferior to acid KMnO4 method for reflecting soil potential nitrogen supplying capacity. The acid KMnO4 method was superior to the alkaline KMnO4 method for reflecting soil present and potential nitrogen supplying capacity and soil total nitrogen supplying capacity. Hot KCl water bath method was equal to the acid permanganate method for reflecting calcareous soil nitrogen supplying capacity, and was superior to the boiling KCl method. The acid KCl method which included initial nitrate could reflect soil total nitrogen supplying capacity very well, but not suitable for reflecting soil potential nitrogen supplying capacity. The boiling water method and NaHCO3 UV method are not suitable for reflecting calcareous soil nitrogen supplying capacity. In a word, the acid KMnO4 method was the best one for reflecting calcareous soil nitrogen supplying capacity. The hot KCl water bath method and H2SO4-KMnO4 method were also better than other chemical methods for reflecting soil total nitrogen supplying capacity (with including soil initial nitrate) and soil potential nitrogen supplying capacity(without including soil initial nitrate).3. Microbial biomass carbon (BC) and microbial biomass nitrogen (BN) had remarkably positive correlations with total nitrogen (TN) and organic matter (OC) (Correlation coefficient were 0.846 and 0.683, respectively, p<0.01), which showed that BC and BN were highly related with soil fertility and they could be used as indexes of biological activity. BC/BN changed from 0.8 to 8.6, which was lower than C/N (5.21~11.89). Therefore, BN was important nitrogen source and pool of the usable nutrient of plant growing. BC and BN were highly correlated with N0 (Correlation coefficient were 0.741 and 0.665, respectively, p<0.01). BN could be used as another index of soil nitrogen supplying capacity. But the values of BN were lower than N0. The average value of the former was 35.5μg.g-1, and that of the latter was 189μg.g-1. BC, BN, TN, OC and N0 all had remarkably positive correlations with physical clay (<0.01mm), but had negative correlations with physical sand (>0.01mm). And significant positive correlations existed between BC, BN, TN, OC, N0 with ratio of physical clay to physical sand (<0.01mm/>0.01mm). The results suggested that soil organic matter was connected with physical clay to form Organic-mineral Complexes. 4. There were significantly differences in the values of microbial biomass carbon (BC) , microbial biomass nitrogen (BN) and nitrogen mineralization potential (N0) in different soil types. The values of BC, BN and N0 were declined from Guanzhong plain to the sandy regions in northern Shaanxi. The values of BC, BN and N0 were highest in Eum Orthic Anthrosols, and were lowest in Hap Ustic Isohumisols, and those were in the middle in Los Orthic Entisols and Ust Sandic Entisols. The values of BC in Eum Orthic Anthrosols, Hap Ustic Isohumisols, Los Orthic Entisols and Ust Sandic Entisols were respectively 305.2μg.g-1, 108.4μg.g-1, 161.7μg.g-1and 125.4μg.g-1; the values of BN were respectively 43.8μg.g-1, 20.3μg.g-1, 26.0μg.g-1and 30.6μg.g-1; the values of N0 were respectively 223μg.g-1, 75μg.g-1, 163μg.g-1and 193μg.g-1. The values of mineralization rate constant (k) were highest in Hap Ustic Isohumisols, and were lowest in Ust Sandic Entisols, and were in the middle in Eum Orthic Anthrosols and Los Orthic Entisols. The values of k in Eum Orthic Anthrosols, Hap Ustic Isohumisols, Los Orthic Entisols and Ust Sandic Entisols were respectively 0.039w-1, 0.044w-1, 0.031w-1 and 0.019w-1. The differences of BC, BN and N0 in mostly soil types on Loess Plateau were resulted from the process of soils forming, plant assimilation products and soil organic matter into soils. The results of the study have evaluative values to analysis the process of soil fertility form on Loess Plateau.
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