The Study On Fixed Ammonium In Major Types Of Upland Soils And Paddy Soils In Hunan Province, China

The content of fixed ammonium, maximum capacity of fixing ammonium and their influencing factors, kinetics of ammonium fixation and release of fixed ammonium, and the contribution of mineral fixation for added NH₄⁺ to the N--maintaining capacity in major types of cultivated upland soils and paddy soils derived from chief parent materials in Hunan province, China, were studied with sampling on fields, and laboratory incubation and determination. 3 of the paddy soils, i.e., yellow clayey soil, purple clayey soil and gray clayey soil were used to study the seasonal change of fixed ammonium contents in the soils treated with different fertilizations and the relationship between the releasing amount and N uptake by rice plant during the rice growth period with pot experiment. Also, 1 of the upland soils, i.e., cultivated brown limestone soil, was used to study the seasonal change of fixed ammonium contents in the soils treated with different fertilizations and the relationship between the releasing amount and N uptake by Huanghuacai, a perennial vegetable crop, during the crop growth period with plot experiment. The main results are summarized as follows:1. Content of fixed ammonium in cultivated horizon of the tested soils varies greatly with nature of parent material and soil use pattern. For the paddy soils, it ranges from 135.4 ± 57.4 mg/kg to 412.8 ±32.4,mg/kg, with 304.7±96.7 mg/kg in average, the percentage of fixed ammonium to total N ranges from 2.6%-25.7%, with 14.0%±5.1% in average. The order of fixed ammonium in the paddy soils in magnitude is newly laky clayey soil>yellow clayey soil>alluvial sandy soil >purple clayey soil >gray clayey soil >reddish yellow clayey soil >granitic sandy soil. For the upland soils, it ranges from 59.4 mg/kg to 435.7 mg/kg, with 230.1 ± 89.2 mg/kg in average, and the percentage of fixed ammonium to total N ranges from 4.6% to 60.1%, with 23.5 ± 14.2% in average. The content of fixed ammonium is in order of cultivated red soil derived from weathering product of slate > cultivated fluviogenic soil derived from limnic material > cultivated purple soil > cultivated fluviogenic soil derived from river alluvium > cultivated red soil derived from weatheringproduct of sandstone > cultivated brown limestone soil > cultivated red soil derived from Quaternary Red Clay > cultivated red soil derived from weathering product of limestone > cultivated red soil derived from weathering product of granite.2. The vertical change of fixed ammonium in profiles of the test soils varies with the soil type. There are 3 change patterns in profiles of the paddy soils, i.e., fixed ammonium content increases with increasing depth in the profile of alluvial sandy soil; it decreases with increasing depth in the profiles of gray clayey soil, reddish yellow clayey soil and granitic sandy soil; it is maximum or minimum in subsurface horizon of the profiles of yellow clayey soil, purple clayey soil and newly laky clayey soil. The percentage of fixed ammonium to total N in soils always increases with the increase of depth: 15.1%±6.6%, 25.9%±11.7% and 34.6%±16.3% in average in the surface horizon, in the subsurface horizon and in the third horizon, respectively. There are only 2 change patterns in profiles of the upland soils, i.e., In the profile of 0⁴0 centimeter depth, the content of fixed ammonium is little changed, and it increases or has no distinct change in the profile of 40 75 centimeter depth. The percentage of fixed ammonium to total N in soils steadily increases with increasing of depth: 20.6%%, 32.4%% and 48.2%% in average in the surface horizon, in the subsurface horizon and in the third horizon, respectively, indicating fixed ammonium is not only important to the N nutrition of shallow root crop, but also more important to the N nutrition of deep root crop.3. Maximum capacity of ammonium fixation in cultivated horizon of the tested soils also varies greatly with nature of parent material and soil use pattern. For the paddy soils, it ranges from 199.5±94.3mg/kg to 541.1±86.0mg/kg. The maximum capacity of ammonium fixation of the soils is in order of purple clayey soil >yellow clayey soil>alluvial sandy soil > gray clayey soil >reddish yellow clayey soil >granitic sandy soil. In profile of the paddy soils, just the maximum capacity of ammonium fixation increased with increasing depth in profiles of yellow clayey soil and purple clayey soil, while the maximum capacity of ammonium fixation of subsurface horizon is the greatest in all other soil profiles. The difference in the maximum capacity of ammonium fixation of different horizon is less than that in content of fixed ammonium of different horizon in the same profile. For theiupland soils, it ranges from 206.6 mg/kg to 533.9mg/kg. The maximum capacity of ammonium fixation is in order of cultivated red soil derived from weathering product of slate > cultivated purple soil> cultivated fluviogenic soil derived from limnic material > cultivated fluviogenic soil derived from river alluvium > cultivated brown limestone soil > cultivated red soil derived fromweathering product of sandstone > cultivated red soil derived from Quaternary Red Clay> cultivated red soil derived from weathering product of limestone > cultivated red soil derived from weathering product of granite. In profiles of the soils, the maximum capacity of ammonium fixation changes vertically at random. Generally the difference in the maximum capacity of ammonium fixation of different horizon in profiles of the upland soils is less than that of the paddy soils because of weaker leaching in profiles of the upland soils. For all the tested soils, % Of recently fixed ammonium to maximum capacity of ammonium fixation is always bellow 20% because of better fertility and higher saturation of ammonium-fixing site of the soils probably.4. The clay content and clay composition of the tested soils are the two important factors influencing their fixed ammonium content and maximum capacity of ammonium fixation. The statistic analysis showed that, Both content of fixed ammonium and maximum capacity of ammonium fixation of the paddy soils were positively correlated with content of 0.01-O.OOlmm clay in the soils at the significant level of pO.Ol, but were not correlated significantly with organic matter, total N, alkali-hydrolyzable N, CEC, slowly released potassium and soil O.OOlmm clay, respectively. The results also showed that the 2:1 type clay mineral in <0.02mm clay of the paddy soils is predominant in hydrous-mica. Of which, the hydrous mica content of 0.02-0.002mm clay is much higher than that of <0.002mm clay of the soils. The statistic analysis showed that both fixed ammonium content and maximum capacity of ammonium fixation of the paddy soils were positively correlated with the total 2:1 type clay mineral content and hydrous mica content of <0.02mm clay and 0.02-0.002mm clay in the paddy soils at the significant level of pO.01 or pO.05, but not correlated with the total 2:1 type clay mineral content and hydrous mica content of <0.002mm clay significantly, respectively. It demonstrated that the ammonium-fixing matrix of the tested soils is mainly hydrous mica in the <0.02mm clay.5. The fixing amounts for added NEU+ in the tested soils all increased with increasing fixation time, the fixing rate for added NH4+ was quick at the initial stage, especially at the initial 12 hours of reaction the fixation process is more fast, and it gradually slowed afterward. After 24 hours, the fixing for added NH4+ reached at equilibrium basically. The kinetics curve belongs to the "L" curve with high affinity. Both first order equation and Elovich equation were the most adaptable to describing the data on NH/- fixation kinetics of the tested soils characterized by the larger correlation coefficients(r) and the lower value of the standard error of estimate (SE). The parabolic diffusion equation could also describethe experimental data satisfactorily. Zero-level equation was not adaptable. The kinetic parameters, the theoretical maximum fixation capacity (A), the reaction rate constant (b) and the half life of ammonium fixation in first-order equation, varies with soil types. Of which, the theoretical maximum fixation capacity(A) and the reaction rate constant(b) of the paddy soils are larger than that of the upland soils.6. Surface adsorption and interlayer-fixation by soil for added ammonium are two important M-maintaining mechanisms of the tested soils. Both account for over 90% of total N-maintaining capacity. Of which, interlayer-fixation by soil for added ammonium amounts to 32.7%, and surface adsorption by soil for added ammonium account for 62.1% in average. But the contribution of interlayer-fixation by soil for added ammonium to the N-maintaining capacity in the tested soils varies with soil types. Fixation for added ammonium by alluvial sandy soil had the greatest contribution to the N-maintaining in soil,. and accounted for 51.5% of the total N-maintaining capacity. Fixation for added ammonium by reddish yellow clayey soil was the lowest, and only accounted for 17. 1%. The order of other five soils in magnitude was yellow clayey soil (43.7%), newly laky clayey soil (35.5%), purple clayey soil (35.4%), gray clayey soil (25.0%), and granitic sandy soil (20.8%), respectively.7. Release of fixed ammonium in the tested soils increased with the incubation time, and the rate of fixed-NH4+-release was very slow. The release of fixed ammonium still continued at the reaction time of 8 weeks for the paddy soils and of 7weeks for the upland soils still had not reached the maximum releasing capacity. The release rate pf fixed ammonium did not change almost during the whole incubation period except the initial 2 days. The releasing rate of recently fixed ammonium is much larger than that of inherently fixed ammonium. For the paddy soils, recently fixed ammonium could release 58.0% to 80.8% of it, and inherently fixed ammonium only released 2.4% to 8.8% of it after 8weeks of incubation. The statistical analysis showed that both first-order equation and parabolic diffusion law equation were the most adaptable to describing the data of fixed-NH4+-release kinetics characterized by the larger correlation coefficient(r) and the lowest value of the standard error of estimate (SE). The category of cation could also affect release of fixed-NH4+ from soil. Ca2+could help release of fixed ammonium perhaps as it could expand the space of the interlayer of 2:1 type clay minerals.8. Pot experiment showed that fixed ammonium contents in the paddy soils changed constantly during rice growth because of fertilization and rice growth. Applying N fertilizer promoted the ammonium fixation, but rice growth promoted release of fixed ammonium.During early rice growth fixed ammonium changed very gently, only recently fixed ammonium released except for CK treatment of yellow clayey soil and its release accounted for 18.8%—100%. But during late rice growth, fixed ammonium changed dramatically. Not only most of recently fixed ammonium released but also did inherently fixed ammonium release partly, too. its release amount accounted for 2.0%-17.7% of inherently fixed ammonium content. There was also significant correlation between the net release rate of fixed ammonium in soils and N uptake by rice plant during the late rice growth.9. Field plot experiment showed that fixed ammonium contents in the soils treated with different fertilizations all increased at the .earlier and middle stages because of applying N fertilizer and mineralization of organic matter, and release of fixed ammonium in the soils happened at the later growth stage of Huanghuacai. During the growth of Huanghuacai recently fixed ammonium in the soils almost released wholly, while inherently fixed ammonium just released in part except the soil treated with NPK+M. Change of fixed ammonium content during the growth stage of Huanghuacai was similar with that of alkali-hydrolyzable N content in the soils. Total N uptake and the biomass of Huanghuacai were positively correlated with the net supplying of soil N (include release of fixed ammonium and applying N) at the significant level of Poos, respectively.