Effects of Glucose Addition on N Transformations in Paddy Soils with a Gradient of Organic C Content in Subtropical China

To better understand the interaction of N transformation and exogenous C source and manage N fertilization, the effects of glucose addition on N transformation were determined in paddy soils with a gradient of soil organic C content. Changes in N mineralization, nitrification and denitrification, as well as their response to glucose addition were measured by incubation experiments in paddy soils derived from Quaternary red clay in subtropical China. Mineralization and denitrification were changed in order of increasing soil fertilities： high 〉 middle 〉 low. During the first week of incubation, net N mineralization and denitrification rates in paddy soil with high fertility were 1.9 and 1.1 times of those in soil with middle fertility and 5.3 and 2.9 times of those in soil with low fertility, respectively. Addition of glucose decreased net N mineralization by approximately 78.8, 109.2 and 177.4% in soils with high, middle and low fertility, respectively. However, denitrification rates in soils with middle and low fertility were increased by 14.4 and 166.2% respectively. The highest nitrate content among the paddy soils tested was 0.62 mg kg-1 and the highest nitrification ratio was 0.33%. Addition of glucose had no obvious effects on nitrate content and nitrification ratio. It was suggested that the intensity of mineralization and denitrification was quite different in soils with different fertility, and increased with increasing soil organic C content. Addition of glucose decreased mineralization, but increased denitrification, and the shifts were greater in soil with low than in soil with high organic C content. Neither addition of glucose nor inherent soil organic C had obvious effects on nitrification in paddy soils tested.

Changes in vegetation and soil properties during recovery of a subtropical forest in South China

Secondary forests account for a large amount of subtropical forest due to persistent anthropogenic disturbance in China.The interaction between vegetation and soil during recovery process is rather complex and dependent on forest conditions.Understanding how vegetation and soil properties changes and how their relationship develops in secondary forests is key to effective forest restoration and management.Here we explored the patterns of vegetation and soil properties as well as their correlations during forest recovery process in a subtropical forest in south China.Plots of three forest types,i.e.,broadleaf-conifermixedforest,broadleaved forest and old growth stand,were established to represent the recovery stages.The results showed that diversity patterns in the tree,shrub and herb layers were different:in the tree layer the species diversity peaked at the intermediate stage,while in the understory layers it decreased chronologically.Most of the soil factors showed an increasing trend,and different effects of soil factors were found for the three layers as well as for the two spatial scales.Together,our results suggested that vegetation and soil might be interdependent during the recovery course.Further studies are needed on exploring how vegetation interplays with soil at different scales and how nutrient limitations affects the vegetation development in a chronosequence.

1.4-nm Intergrade Mineral in Soils of Snbtropical China:A Review

This paper is a review of some advances in the studies on 1.4-nm intergrade mineral of soils in sub-tropical China. 1) 1.4-nm intergrade mineral occurs ubiquitously in soils of subtropical China. The 1.4-nmrnineral in red soil and yellow soil is mainly 1.4-nm intergrade mineral, and in acidic yellow-brown soil (pH< 5.5) is verniiculite alone or 1.4-nm intergrade mineral together with vermiculite. The distribution and thecontent of 1 .4-nm intergrade mineral in the mountain soils are more widespread and higher than those of thecorresponding soils in horizontal zone. 2) The interlayer material of 1.4-nin intergrade mineral ui these soilsappears to be hydroxy-Al polymers instead of hydroxy-Fe, proto-imogolite or kaolin-like material. There isa significant positive correlation between Al amount extracted from the soil with sodium citrate after DCBextraction and pH value of the citrate solution after the extraction. The citrate can also extract a certainamount of silicon from the soil, but the silicon may not come from interlayer of 1.4-nm intergrade mineral.3) It was seldom studied that either vermiculite or smectite did the natural 1.4-nm intergrade mineral comefrom in soil, or it was commonly thought to come from vermiculite. A recent report has revealed that itcan come from smectite. There are some different behaviors between the 1.4-nm intergrade mineral derivedfrom vermiculite and that from smectite. For example, they exert different influences on the formation ofgibbsite. The 1.4-nm intergrade mineral derived from smectite may promote the formation of gibbsite in theyellow soil. 4) The type of 1.4-nm minerals in soils. i.e., vermiculite or 1.4-nm intergrade mineral, may besignificant to soil properties, such as soil acidity, exchangeable Al, electric charge aiiiount and specific surfacearea. Therefore, the management for the soil in which 1.4-nm mineral is mainly 1.4-nm intergrade mineralor vermiculite should be dealt with differently.

Phosphate Adsorption and Surface Charge Characteristics and Their Relations to Mineralogy of Some Soils from Southern China

The phosphate adsorption and surface charge characteristics of the tropical and subtropical soils derived from different parent materials in China were determined, and their relations to soil mineralogy were analysed.The results showed that all soil phosphate adsorption curves were well fitted by Freundlich equation and Langmuir equation. The maximum buffering capacity of P ranged from 66 to 9880 mg kg-1, with an increasing order of purple soil, skeletal soil, red soil, lateritic red soil, yellow soil and latosol; and the highest value was 149 times the lowest value, which indicated great differences among these soils in phosphate adsorption and supplying characteristics. The pHo (zero point of charge) values obtained by salt titrationpotential titration varied from 3.03 to 5.49, and the highest value was found in the latosol derived from basalt whereas the lowest value was found in the purple soil. The correlation analysis indicated that the main minerals responsible for phosphate adsorption in the soils were gibbsite, amorphous iron oxide and kaolinite; and the pHo was mainly controlled by kaolinite, gibbsite and oxides.

Soil respiration under three different land use types in a tropical mountain region of China

Soil respiration (SR) Wis one of the largest contributors of terrestrial CO_2 to the atmosphere.Environmental as well as physicochemical parameters influence SR and thus, different land use practices impact the emissions of soil CO_2. In this study, we measured SR, bi-monthly, over a one-year period in a terrace tea plantation, a forest tea plantation and a secondary forest, in a subtropical mountain area in Xishuangbanna, China. Along with the measurement of SR rates, soil characteristics for each of the land use systems were investigated. Soil respiration rates in the different land use systems did not differ significantly during the dry season, ranging from2.7±0.2 μmol m^(-2) s^(-1) to 2.8±0.2 μmol m^(-2) s^(-1). During the wet season, however, SR rates were significantly larger in the terrace tea plantation(5.4±0.5 μmol m^(-2)s^(-1)) and secondary forest(4.9±0.4 μmol m^(-2)s^(-1)) than in the forest tea plantation(3.7±0.2 μmol m^(-2) s^(-1)).This resulted in significantly larger annual soil CO_2 emissions from the terrace tea and secondary forest,than from the forest tea plantation. It is likely that these differences in the SR rates are due to the 0.5times lower soil organic carbon concentrations in thetop mineral soil in the forest tea plantation, compared to the terrace tea plantation and secondary forest.Furthermore, we suggest that the lower sensitivity to temperature variation in the forest tea soil is a result of the lower soil organic carbon concentrations. The higher SR rates in the terrace tea plantation were partly due to weeding events, which caused CO_2 emission peaks that contributed almost 10% to the annual CO_2 flux. Our findings suggest that moving away from heavily managed tea plantations towards low-input forest tea can reduce the soil CO_2 emissions from these systems. However, our study is a casestudy and further investigations and upscaling are necessary to show if these findings hold true at a landscape level.

Effect of orchard age on soil nitrogen transformation in subtropical China and implications

A better understanding of nitrogen transformation in soils could reveal the capacity for biological inorganic N supply and improve the efficiency of N fertilizers. In this study, a15 N tracing study was carried out to investigate the effects of converting woodland to orchard, and orchard age on the gross rates of N transformation occurring simultaneously in subtropical soils in Eastern China. The results showed that inorganic N supply rate was remained constant with soil organic C and N contents increased after converting woodland into citrus orchard and with increasing orchard age. This phenomenon was most probably due to the increase in the turnover time of recalcitrant organic-N, which increased with decreasing soil p H along with increasing orchard age significantly. The amo A gene copy numbers of both archaeal and bacterial were stimulated by orchard planting and increased with increasing orchard age. The nitrification capacity（defined as the ratio of gross rate of nitrification to total gross rate of mineralization） increased following the Michaelis–Menten equation, sharply in the first 10 years after woodland conversion to orchard, and increased continuously but much more slowly till 30 years. Due to the increase in nitrification capacity and unchanged NO3-consumption, the dominance of ammonium in inorganic N in woodland soil was shifted to nitrate dominance in orchard soils. These results indicated that the risk of NO3-loss was expected to increase and the amount of N needed from fertilizers for fruit growth did not change although soil organic N accumulated with orchard age.

Soil Enzyme Activities and Organic Matter Composition Affected by 26 Years of Continuous Cropping

The study was to determine the long-term effects of subtropical monoculture and rotational cropping systems and fertilization on soil enzyme activities and soil C, N, and P levels. Cropping systems included continuous sorghum(Sorghum bicolor L.), cotton(Gossypium hirsutum L.), corn(Zea mays L.), and cotton/sorghum rotations after 26 years of treatment imposition. Soil under continuous sorghum and continuous corn had 15% and 11%, respectively, greater C concentrations than soil under continuous cotton.Organic C was 10% higher at 0–7.5 cm than at 7.5–15 cm. Total N followed similar trends with soil depth as organic C. Continuous sorghum had 19% higher total N than other crop species and rotations. With fertilization, continuous cotton had the highest total P at 0–7.5 cm and sorghum had the highest at 7.5–15 cm. Soil total P was 14% higher at 0–7.5 than at 7.5–15 cm, and fertilization increased 15% total P compared to unfertilized soil. Arylsulfatase, alkaline phosphatase, and β-d-glucosidase activity were the highest for sorghum and the lowest for cotton. Rotation increased enzyme activities compared to continuous cotton but not for continuous sorghum. Of all crop species and rotations, continuous cotton generally showed the lowest levels of organic matter and enzyme activities after 26 years. Fertilization significantly increased the yields for all cropping systems, but rotation had no significant effect on either sorghum or cotton lint yield compared to each crop grown in monoculture. Long-term cropping did not increase soil organic matter levels beyond short-term gains, indicating the difficulty in promoting C sequestration in subtropical soils.