Role of aggregate surface and core fraction in the sequestration of carbon from dung in a temperate grassland soil

The sequestration of dung carbon in soil depends on the location and rate at which it is immobilized in soil aggregates. Here C₄ dung (δ¹³C = ?16.1‰) or C₃ dung (δ¹³C = ?26.8‰) were applied to a temperate permanent pasture C₃ soil (δ¹³C = ?27.9‰). Triplicate samples were taken from C₃ and C₄ dung remaining at the surface, and in the 0–1 and 1–5 cm soil layers in the unamended control and under the C₃ and C₄ dung patches after 7, 14, 29, 42 and 70 days after the application of the dung. Macroaggregates (≥ 4 mm) at the lower depth (1–5 cm) were mechanically fractionated into surface and core fractions by a combination of shock freezing followed by wet sieving. Neither overall nor differential carbon isotope fractionation occurred in the dung remaining at the surface. The incorporation of C₄ dung significantly increased the δ¹³C content of the 0–1 cm layer of the C₃ soil. Dung C sequestration did not exceed 10% for the 0–1 cm layer and was only 20% for the whole soil (0–30 cm) during the 7‐day experiment. Only 32–66% of the C from dung in the 1–5 cm layer was sequestered in the aggregates; the major proportion was initially preferentially attached to their surfaces, but incorporated into aggregates within the following 14 days. The majority of dung, however, soon resided between the aggregates, pointing to the important role of the inter‐aggregate fraction in short‐term C dynamics of dung in this pasture soil.     

Hot water extractable C and N in relation to microbiological properties of soils under beech forests

Hot water extraction is sometimes recommended as an easy method to estimate the readily mineralizable fractions of total C (C_t) and total N (N_t) in arable soils. However, the usefulness of this method for forest soils has not been adequately studied. The objectives of this study were to relate the hot water extractable C (C_hw) and N (N_hw) to microbiological and chemical properties of the forest soils under beech (Fagus sylvatica L.) stands and to test the ability of near infrared spectroscopy (NIRS) to predict chemical and microbial properties of these soils. Soils differing in humus type, soil type and soil texture were collected from five locations and five depths. In all soils the amount of C_hw was higher than the microbial biomass C (C_mic) indicating that a considerable part of C_hw was of non-microbial origin. The amount of C_hw in mineral soil correlated significantly (r =–0.30–0.53) with C_mic, basal respiration (BAS) and C_t/N_t ratio but was not related to C_mic/C_t ratio. The amount of N_hw was correlated with C_mic, BAS, C_mic/C_t ratio, and C_t/N_t ratio (r =–0.59–0.78). However, C_t and N_t values showed better relationships (r =–0.42–0.88) with all the parameters, indicating no advantage in using C_hw and N_hw in forest soils. NIRS predicted satisfactorily C_t, N_t, C_hw, N_hw, C_mic, C_mic/C_t ratio and BAS in the mineral soils [the regression coefficients (a) of linear regression (measured against predicted values) ranged from 0.84 to 1.17 and the correlation coefficients (r) ranged from 0.86 to 0.94] indicating the applicability of NIRS to estimate these properties.     

Microbial biomass and activity under oxic and anoxic conditions as affected by nitrate additions

Soil microbial activity, biomass, and community structure were examined during the transition from oxic to anoxic conditions after the addition of glucose and with or without nitrate addition. In two sets of treatments, samples were incubated for up to 35 d in closed ampoules either aerobically until oxygen was depleted or anoxically throughout the experiment. Heat‐flow rate was monitored to indicate microbial activity. Microbial biomass and community structure were measured by adenylate and phospholipid fatty acid (PFLA) content, and adenylate energy charge (AEC) was used to monitor the physiological status of the microbial biomass. Microbial activity was highest under oxic conditions and abruptly decreased under anoxic conditions. Activity peaks were observed after about 9 d of anoxic conditions probably triggered by increased nutrient availability from dying microbial biomass, but these peaks were smaller after initial oxic incubation or nitrate addition. Microbial biomass was unchanged under oxic conditions but decreased under anoxic conditions. Most surviving microbes switched into dormancy. Changes in the microbial‐population structure were small and occurred only after 9 d of anoxic incubation. The results show that the nutrient status and the availability of electron acceptors such as nitrate were important factors ruling the direction and the extent of shifts in the microbial activity and community structures due to anoxic conditions.     

Temperature dependence of nitrogen mineralization and microbial status in O<Subscript>H</Subscript> horizon of a temperate forest ecosystem

It was hypothesized that increasing air and/or soil temperature would increase rates of microbial processes including litter decomposition and net N mineralization, resulting in greater sequestration of carbon and nitrogen in humus, and consequently development in OH horizon （humus horizon）. To quantify the effect of temperature on biochemical processes controlling the rate of OH layer development three adjacent forest floors under beech, Norway spruce and mixed species stands were investigated at Soiling forest, Germany by an incubation experiment of OH layer for three months. Comparing the fitted curves for temperature sensitivity of OH layers in relation to net N mineralization revealed positive correlation across all sites. For the whole data set of all stands, a Q10 （temperature sensitivity index） value of 2.35-2.44 dependent on the measured units was found to be adequate for describing the temperature dependency of net N mineralization at experimental site. Species-specific differences of substrate quality did not result in changes in biochemical properties of OH horizon of the forest floors. Temperature elevation increased net N mineralization without significant changes in microbial status in the range of I to 15℃. A low Cmic /Corg （microbial carbon/organic carbon） ratio at 20℃ indicated that the resource availability for decomposers has been restricted as reflected in significant decrease of microbial biomass.     

温带森林系统中碳矿化和氮氧化释放的温度依赖性研究

测定森林土壤中的CO2 和N2O 的释放,在评价森林平衡大气中CO2 和N2O的作用方面有着重要的意义.为了量化立地对净碳矿化和N2O氮释放温度依赖性的影响,在德国的索尔森林,调查了三毛举林,挪威云杉和混合林三种相邻的地被物,并进行了3个月不同温度的培养试验.结果表明,3种森林地被物的净碳矿化率和N2O氮的释放量随温度上升呈现指数性增长.在一定温度范围内(1(20oC),利用温度系数函数(Q10)拟合通量率来描述森林地被物的温度敏感性.各试验点的森林地被物的温度敏感性拟合曲线与净碳矿化和N2O氮释放率都显正相关.各试验林的全部数据表明,每个单位的净碳矿化和N2O氮释放的温度系数函数值(Q10)分别为1.73(2.10和2.81(3.58,可用以描述试验地净碳矿化和N2O氮释放率对温度的依赖性.在三毛举和云杉的单一树种和混合种中,净碳矿化率 和N2O 氮释放率的温度依赖性没有明显的差异,表明净净碳矿化率和N2O 氮释放量不受不同树种凋落物质量的影响.     

氮肥对盆栽欧洲水青冈生物量和营养分配的影响(英文)

调查了施加氮肥(15NH4和15NO3)处理后在两个连续生长季内欧洲水青冈(Fagus sylvatica L.)幼苗地上部分和地下部分的生物量和营养元素分配。盆栽欧洲水青冈幼苗培养于温室大棚内,培养土样取自相邻的三种林分:欧洲水青冈,挪威赤松,欧洲水青冈-赤松混交林。结果表明,氮肥(15N)处理对欧洲水青冈营养元素分配没有显著影响,施加氮素形式决定自身流入植物库的情况。欧洲水青獭收氮素主要以硝态氮的形式,因此,尽管植物体内保存的硝态氮和氨态氮并没有统计差异,但是叶片中硝态氮明显减少。施加硝态氮对欧洲水青冈氮素恢复的影响要大于施加氨态氮。与欧洲水青冈茎、粗根相比,优质根系对氮素(15N)固定是一个缓慢过程。表8图1参40。     

Decomposition of maize residues after manipulation of colonization and its contribution to the soil microbial biomass

A 28-day incubation experiment at 12°C was carried out on the decomposition of maize leaf litter to answer the questions: (1) Is the decomposition process altered by chemical manipulations due to differences in the colonization of maize leaf litter? (2) Do organisms using this maize material contribute significantly to the soil microbial biomass? The extraction of the maize straw reduced its initial microbial biomass C content by 25%. Fumigation and extraction eliminated the microbial biomass by 88%. In total, 17% of added maize straw C was mineralized to CO₂ during the 28-day incubation at 12°C in the treatment with non-manipulated straw. Only 14% of added C was mineralized in the treatment with extracted straw as well as in the treatment with fumigated and extracted straw. The net increase in microbial biomass C was 79 μg g^?1 soil in the treatment with non-manipulated straw and an insignificant 9 μg g^?1 soil in the two treatments with manipulated straw. However, the net increase did not reflect the fact that the addition of maize straw replaced an identical 58% (≈180 μg g^?1 soil) of the autochthonous microbial biomass C₃-C in all three straw treatments. In the two treatments with manipulated straw, the formation of maize-derived microbial biomass C₄-C was significantly reduced by 25%. In the three straw treatments, the ratio of fungal ergosterol-to-microbial biomass C ratio showed a constant 60% increase compared to the control, and the contents of glucosamine and muramic acid increased by 18%. The average fungal C/bacterial C ratio was 3.6 in the soil and 5.0 in the recovered maize straw, indicating that fungal dominance was not altered by the initial chemical manipulations of the maize straw-colonizing microorganisms.