Nutrient losses from rain‐fed bench terraced cultivation systems in high rainfall areas of the mid‐hills of Nepal

Between the elevations of 1000 and 2000 m in the mid‐hills of Nepal, over 12 million people subsist on land‐holdings of less than 0·5 ha. These farmers have limited access to commercial inputs such as fertilisers and are reliant on organic manures for soil fertility maintenance. Participatory research was conducted with farmers on bari land (upper slope rain‐fed crop terraces) in the hill community of Landruk (bench terraces 0–5° slope, 3000–3500 mm annual rainfall, which aimed to develop soil and water management interventions that controlled erosion without resulting in high leaching, and so were effective in minimising total nutrient losses. Interventions tested were the control of water movement through diversion of run‐on and planting fodder grasses on terrace risers on bench terraces. The interventions were effective in reducing soil loss from the bari land in comparison with existing farmer practices, but no effect was observed on nutrient losses in solution form through runoff and leaching. Losses of NO₃‐N in leachate ranged from 17·3 to 99·7 kg ha⁻¹ yr⁻¹, but only 0·7 to 5·6 kg ha⁻¹ yr⁻¹ in runoff. The overall nutrient balance suggests that the system is not sustainable. Fertility is heavily dependent on livestock inputs and if the current trends of declining livestock numbers due to labour constraints continue, further losses in productivity can be expected. However, farmers are interested in interventions that tie ecosystem services with productivity enhancement and farmers' priorities should be used as entry points for promoting interventions that are system compatible and harness niche opportunities. Copyright © 2007 John Wiley & Sons, Ltd.     

Influence of erosion on soil microbial biomass,abundance and community diversity

This study aimed to determine microbial biomass carbon and microbial abundance immediately after, and two years after, forest soil erosion, so as to estimate the degree of damage, including the rate of recovery of microorganisms, in each area. It also aimed to determine the community diversity, and to establish relationships between microbial biomass, microbial abundance and the physico‐chemical properties of the soil. Three different study areas in Hiroshima Prefecture, Japan, were used. One undisturbed area and two eroded areas (one immediately after and one two years after erosion). The analysis of variance showed a highly significant difference in microbial biomass carbon and abundance between the study areas. The undisturbed area showed the highest value, followed by the area eroded two years ago, then lastly the area studied immediately after the erosion. The biomass carbon was highly correlated with gram positive bacteria with r² = 0·983, p < 0·01. The biomass carbon and microbial population were shown to be significantly correlated to the soil's physico‐chemical properties, such as pH, moisture content, water‐holding capacity and CN ratio. However, CN ratio proved to be closely correlated to biomass carbon with r² = −0·978, p < 0·01, to Gram‐positive bacteria with r² = −0·977, p < 0·01, to Gram‐negative bacteria with r² = −0·989, p < 0·01 and to fungi with r² = −0·977, p < 0·01. The undisturbed area showed a highly diverse community in both of the restriction enzymes used, followed by the area affected by erosion two years ago, then the area immediately after erosion. Copyright © 2004 John Wiley & Sons, Ltd.     

红壤稻田土壤有机质的积累过程特征分析

通过田间采样分析 ,研究了不同利用年限红壤稻田土壤有机质含量的变化及其过程和机理 ,确定达到平衡状态时红壤稻田土壤的有机碳含量水平。结果表明 ,在水耕条件下 ,土壤有机碳和全氮的积累过程可大致分为快速增长和趋于稳定阶段 ,水耕利用 30年 ,0～ 2 0cm土壤有机碳含量达到 2 0gkg- 1,全氮含量 1 6gkg- 1,随后 ,即使利用年限长达 80年 ,土壤有机碳和全氮含量变化趋于稳定 ,没有显著提高。 2 0天的培养期内 ,不同利用年限红壤稻田 0～ 1 0cm土层有机碳和有机氮的矿化率分别为 2 2 %～3 3%和 2 8%～ 6 7% ;总体来说 ,有机碳、氮的矿化率随红壤水稻土的熟化过程而升高。随着利用年限的增加 ,微生物生物量碳一直保持增加的趋势 ,而微生物生物量氮在利用 30年后其增加趋势明显趋缓 ;利用30年的红壤稻田 ,0～ 1 0cm土壤微生物生物量C、N为 332 8mgkg- 1和 2 3 85mgkg- 1,比利用 3年分别高1 1 1 %和 4 7%。与利用 3年的红壤稻田相比 ,利用 30年后细菌数量增加了 1 1倍 ( 0～ 1 0cm)和 3 8倍 ( 1 0～2 0cm) ,利用 80年后更显著地增加了 1 9倍 ( 0～ 1 0cm)和 1 2倍 ( 1 0～ 2 0cm) ;真菌的数量也呈上升的趋势 ,但在 30年利用后基本趋于稳定 ;此外 ,细菌的群落从荒草地的 4个种到 30～ 80年水田     

红壤水稻土肥力性状的演变特征

大田条件下 ,通过选点采样分析 ,研究了不同利用年限红壤水稻土的物理、化学和生物学性质的动态变化特征。荒地红壤水耕利用后 ,土壤颗粒组成呈现规律性变化 ,粘粒 (<0 0 0 2mm)含量从荒地红壤的 3 9%下降到 80a稻田土壤的 1 7% ,而粉砂 (0 0 2～ 0 0 0 2mm)含量升高。土壤pH一般增加 0 5～ 1个单位 ;0～ 1 0cm土壤有机碳和全氮含量从荒地红壤的 4 5 8gkg- 1和 0 3 9gkg- 1增加到 3 0a红壤稻田的1 9 6gkg- 1和 1 62gkg- 1,其后 ,即使利用时间长达 80a ,土壤有机碳和全氮含量并没有显著差异 ;土壤全磷含量 ,经 3a水耕利用后可从荒地红壤的 0 5gkg- 1提高到 1 3gkg- 1,表明通过施肥可使红壤磷素快速积累 ;而在水耕利用过程中 ,红壤稻田土壤的钾素含量呈下降趋势 ,经 80a利用的红壤稻田 ,0～ 1 0cm土壤钾素含量仅为荒地红壤的 80 % ;全铁和游离铁的含量也呈下降趋势。随着水耕熟化过程 ,细菌数量和脲酶活性也明显升高。不同利用年限红壤稻田土壤的各项性状指标的变化结果还表明 ,荒地红壤水耕利用后要达到高度熟化的稻田土壤肥力水平 ,需要经过 3 0a的时间     

VARIATION IN SOIL ORGANIC CARBON AND NITROGEN STOCKS ALONG A TOPOSEQUENCE IN A TRADITIONAL MEDITERRANEAN OLIVE GROVE

The impact of the topographical position on soil properties was evaluated in an olive grove with traditional tillage. Three topographical positions: summit, backslope and toeslope were chosen for evaluation. The soil samples were taken from four soil sections of 0·25 m (0–1 m). The soil organic carbon (SOC) and N content increased along the downslope direction (5·5, 6·5 and 7·1 g C kg⁻¹ and 0·3, 0·8 and 0·9 g N kg⁻¹ in the surface layer in the summit, backslope and toeslope respectively) as well as SOC and N stocks, considering the two first soil sections. In addition, there was movement of the most erodible textural fraction (silt). However, the total SOC stock (refer to 1 m of depth) did not vary with respect to the topographical position, but the total N stock (refer to 1 m of depth) varied significantly. These increases were due to erosion processes that occur along the toposequence, leading to organic matter transfers from the summit to the toeslope. All the stratification ratios calculated were lower than 2, indicating the low quality of the soils. Therefore, alternative management techniques that avoid soil erosion must be considered in olive grove in order to increase the soil quality and fertility. Copyright © 2014 John Wiley & Sons, Ltd.     

EFFECTS OF CRUSHED MAIZE STRAW RESIDUES ON SOIL BIOLOGICAL PROPERTIES AND SOIL RESTORATION

Agriculture soils in the Mediterranean need restoration and rehabilitation after 10 millenia of use and abuse. Maize straw residues crushed at three sizes [<1 (C1), 1–10 (C2) and >10 cm (C3)] at 5 Mg ha⁻¹ y⁻¹ and with and without urea (150 kg N ha⁻¹) were applied during a period of 3 years for the purpose of restoration of a Typic Xerofluvent located near Córdoba (Spain). The effect on the vegetal cover and biological properties (microbial biomass, soil respiration and enzymatic activities) were determined. The size of the crushed maize residues (particle size <1 cm) and N supply influenced in the evolution of soil biological properties and vegetal cover. The stimulation of microbial biomass, soil respiration, dehydrogenase, urease, β‐glucosidase, phosphatase and arylsulphatase activities was higher in C1 + N‐amended soils for 14·9%, 16·3%, 8·8%, 24·3%, 13·5%, 7·1% and 10·3%, respectively, compared with C2 + N and for 25·8%, 26·9%, 18·3%, 38·5%, 28·2%, 19·1% and 18·3%, respectively, compared with C3 + N. Vegetal cover from the C1 + N treatment was 11·4%, 17·8%, 29·4%, 37·6%, 44·9% and 75·1% greater than that in C2 + N, C3 + N, C1, C2, C3 and control soil. These results suggested that under dry climatic conditions, the application of crushed maize straw finely crushed + N fertilizers improved the soil biological properties and also favour the appearance of spontaneous vegetation, which will protect the soil and will contribute to its restoration. Consequently, the addition of the crushed maize straw finely crushed + N fertilizers may be considered a good environmental strategy for recovery of degraded soils. Copyright © 2014 John Wiley & Sons, Ltd.     

A Review of the Use of Manure in Small-Scale Crop Production Systems in South Africa

ABSTRACT

This paper reviews the problems of soil fertility under small-scale crop production systems in KwaZulu-Natal province in South Africa. The role of manure in maintaining and replenishing soil fertility for crop production and the fate of manure once applied to the soil are reviewed and discussed. Special emphasis is placed on cattle and chicken manure and the role that soil texture plays in the mineralization of nitrogen (N) and phosphorus (P). In KwaZulu-Natal, small-scale farmers' maize (Zea mays L.) yields are between 1.0 and 1.5 mg ha^?1, which is very low compared with the maize potential yield of 4.5 mg ha^?1 for the area under small-scale farming conditions. A review of available literature on the use of manure for soil-fertility management showed that manure is a good source of plant nutrients. The use of manure is an old technology that is appropriate for small-scale farmers in South Africa, as most farmers practice mixed livestock and crop farming. Despite the use of manure dating back many years, small-scale farmers in South Africa are not fully exploiting the available manure for replenishing the fertility of their soils.     

EXCLOSURE LAND MANAGEMENT FOR RESTORATION OF THE SOILS IN DEGRADED COMMUNAL GRAZING LANDS IN NORTHERN ETHIOPIA

In the northern highlands of Ethiopia, establishment of exclosures to restore degraded communal grazing lands has been practiced for the past three decades. However, empirical data on the effectiveness of exclosures in restoring degraded soils are lacking. We investigated the influence of exclosure age on degree of restoration of degraded soil and identified easily measurable biophysical and management‐related factors that can be used to predict soil nutrient restoration. We selected replicated (n = 3) 5‐, 10‐, 15‐, and 20‐year‐old exclosures and paired each exclosure with samples from adjacent communal grazing lands. All exclosures showed higher total soil nitrogen (N), available phosphorus (P), and cation exchange capacity than the communal grazing lands. The differences varied between 2·4 (±0·61) and 6·9 (±1·85) Mg ha⁻¹ for the total N stock and from 17 (±3) to 39 (±7) kg ha⁻¹ for the available P stock. The differences in N and P increased with exclosure age. In exclosures, much of the variability in soil N (R² = 0·64) and P (R² = 0·71) stocks were explained by a combination of annual average precipitation, woody biomass, and exclosure age. Precipitation and vegetation canopy cover also explained much of the variability in soil N (R² = 0·74) and P (R² = 0·52) stocks in communal grazing lands. Converting degraded communal grazing lands into exclosures is a viable option to restore degraded soils. Our results also confirm that the possibility to predict the changes in soil nutrient content after exclosure establishment using regression models is based on field measurements. Copyright © 2011 John Wiley & Sons, Ltd.     

Assessing Farmers' Knowledge of Weed Species,Crop Type and Soil Management Practices in Relation to Soil Quality Status in Mai‐Negus Catchment,Northern Ethiopia

Soil quality (SQ) assessment from farmers' point of view can be used as a primary indicator for planning sustainable agriculture. Despite this fact, limited information is documented with regard to SQ indicators, for example weed species, crop types and management practices from farmers' knowledge perspectives. The aims of this study are to analyse factors that determine farmers' knowledge of SQ, identify SQ indicators of weed species and crop types across different SQ status and assess soil‐and‐crop management practices that maintain SQ in the Mai‐Negus catchment, northern Ethiopia. Fifty‐two farmer household heads were chosen randomly for questionnaire interview. The results showed significant (p ≤ 0·05) differences in the proportion of respondents who used different crop‐and‐soil management practices. The success of overall prediction by the logistic regression model (model χ² = 0·84, p < 0·01) and level of model correct predictions (86%) indicated that the explanatory variables have sufficiently explained farmers' knowledge of SQ indicators. Provided that other conditions remained constant, the odds ratio of variables such as farmer experience, access to information, farm location, education, field slope and land tenure have significantly increased the likelihood of farmers for being knowledgeable of SQ indicators. This study also demonstrated that most farmers are knowledgeable in identifying weed species, crop types and management practices across various SQ status, which suggests that such SQ indicators should be used to assess SQ status (degradation severity) while locating fields to be intervened using appropriate management strategies. © 2013 The Authors. Land Degradation & Development published by John Wiley & Sons, Ltd.     

INORGANIC SOIL PHOSPHORUS

Sixteen soils and 4 soil preparations were cropped exhaustively with ryegrass in the glasshouse and monocalcium phosphate potentials (½pCa+pH₂PO₄=1) were measured after each of 6 consecutive harvests. The amounts of phosphorus (Q) removed from the soils by ryegrass accounted for 95·1–96·6 per cent of the variance in 1 for 3 soils and 2 soil preparations (P < 0·001), for 88·4–93·7 Per cent of the variance for 6 soils and 2 soil preparations (0·001 < P < 0·01), for 71·6–82·6 per cent of the variance for 3 soils (0·01 < P < 0·05) and for insignificant amounts of the variance for 4 soils. Values of ΔI/ΔQ ranged from 7 × 10^–4 to 431 × 10^–4½pCa+pH₂PO₄/ppm P removed from soil. ΔI/ΔQ tended to decrease (i.e. the soils were more buffered) with increasing clay contents and with increasing amounts of NaHCO₃-soluble P and to increase (i.e. the soils were less buffered) with increasing amounts of CaCO₃. Variations in organic C did not significantly affect ΔI/ΔQ. The following equation accounts for 81 per cent of the variance in ΔI/ΔQ for all soils except those in equilibrium with octacalcium phosphate: ΔI/ΔQ× (10⁴) = 225·9–4·17(% clay)+8·01(% CaCO₃)–1·38(ppm NaHCO₃-soluble P).     

Effectiveness OF Mulching Under Miraba in Controlling Soil Erosion,Fertility Restoration and Crop Yield in the Usambara Mountains,Tanzania

Soil erosion is a major threat to food security in rural areas of Africa. Field experiments were conducted from 2011 to 2014 in Majulai and Migambo villages with contrasting climatic conditions in Usambara Mountains, Tanzania. The aim was to investigate the effectiveness of mulching in reducing soil erosion and restoring soil fertility for productivity of maize (Zea mays) and beans (Phaseolus vulgaris) under miraba, a unique indigenous soil conservation measure in the area. Soil loss was significantly higher (p < 0·05) under miraba sole than under miraba with mulching, for example, 35 versus 20 and 13 versus 8 Mg ha⁻¹ y⁻¹ for Majulai and Migambo villages, respectively, in 2012. Soil fertility status was significantly higher (p < 0·05) under miraba with Tughutu mulching than under miraba sole, for example, 0·35 versus 0·25% total N, 37 versus 22 mg kg⁻¹ P and 0·6 versus 0·2 cmol(+) kg⁻¹ K for the Majulai village; and 0·46 versus 0·38 total N, 17·2 versus 10·2 mg kg⁻¹ P and 0·50 versus 0·2 cmol(+) kg⁻¹ K for the Migambo village. Maize and bean yields (Mg ha⁻¹) were significantly higher (p < 0·05) under miraba with Tughutu mulching than under miraba sole, 2·0 versus 1·3 for maize and 0·9 versus 0·8 for beans in Majulai; and 3·8 versus 2·6 for maize and 1·0 versus 0·8 for beans in the Migambo village in 2012. This implies that Tughutu mulching is more effective in improving crop yield than Tithonia, although both could potentially protect the arable land from degradation caused by water erosion under miraba. Copyright © 2014 John Wiley & Sons, Ltd.     

Optimizing the Nitrogen Management Strategy for Winter Wheat in the North China Plain Using Rapid Soil and Plant Nitrogen Measurements

Excessive nitrogen (N) fertilizer with improper split-application in small-scale farming is widespread for reducing N use efficiency and polluting the environment. The objective of this study was to develop a strategy for providing winter wheat with twice-topdressing N by quickly measuring the soil and plant N status. During the period 2009–2011, a field experiment was conducted for winter wheat cultivar Zhongmai-175 in the North China Plain. The mineral N (Nmin) pool at a soil depth of 0–90 cm and topdressing N twice, as total N supply, was gradually increased from 0 to 420 kg N ha^–1 to mimic the farmers´ practices. Measurements with the Soil Plant Analysis Development (SPAD) meter were taken on the uppermost fully expanded leaf, and the SPAD index was expressed relative to SPAD readings of sufficiently fertilized plants. Grain yield exhibited linear-plus-plateau responses to total N supply with a significant difference between years, the r² ranged from 0.73 to 0.94. With a basal N application of 30 kg ha^–1, the soil Nmin at 0–90 cm supplemented by twice-topdressing N (1:1 ratio) at Zadoks growth stage (ZGS) 22–23 in early spring and ZGS 47–52 was required at 150–165 kg N ha^–1 to achieve a maximum grain yield of 3.9–5.3 t ha^–1. The SPAD index exhibited a strong exponential response to N supply irrespective of plant growth stage and year (r² = 0.95–0.97); the value of 0.94 was critical in denoting N deficiency from sufficiency status. The N topdressing at ZGS 47–52 could be precisely modified/estimated by the equation y = 161.7–218x^5.16, where x is the SPAD index. Since SPAD readings varied significantly from year to year, our study suggests that it might be difficult to precisely manage field N for winter wheat.     

Using farmer knowledge to combat low productive spots in rice fields of a Sahelian irrigation scheme

In the oldest sections of Burkina Faso's largest irrigation scheme in the Sourou Valley (13° 10′ N, 03° 30′W) rice (Oryza sativa L.) yields dropped from about 5 to 6 t ha⁻¹ in the early 1990s, shortly after establishment of the scheme, to 2 to 4 t ha⁻¹ from 1995 onwards. Farmers blamed this yield decline on the appearance of 2 to 20 m diameter low productive spots. According to farmers and field measurements, the low productive spots decreased yields by 25–50 per cent. The low productive spots are caused by Zn deficiency. Low Zn availability is related to the very low DTPA‐extractable Zn content of the soil (0·08–0·46 mg kg⁻¹), the alkaline‐calcareous character of the soil, the non‐application of Zn fertilizers, and a relatively large P fertilizer dose (21 kg P ha⁻¹). Farmers were correct in relating the calcareous nature of the soil to the presence of the low productive spots. They were instrumental in identifying application of decomposed organic resources (e.g. rice straw at 5 t ha⁻¹) as a short‐term solution that increases yields by 1·5 to 2·0 t ha⁻¹. Application of Zn fertilizer (10 kg Zn ha⁻¹) in 29 farmer fields in the 2001 dry season eradicated the low productive spots and increased yields from 3·4 to 6·0 t ha⁻¹. Although application of Zn fertilizer is strongly recommended, it is not yet available in Burkina Faso. Based on a comparison of fertilizer prices on the world market and the local market, we expect that the use of Zn fertilizers will be highly profitable (cost/value ratio ≫ 2). Despite the relatively recent introduction of irrigated rice cropping, most farmers showed a good understanding of cropping constraints and possible solutions. Both farmers and researchers mutually benefited from each other's knowledge and observations. Copyright © 2004 John Wiley & Sons, Ltd.     

黄土丘陵区人工刺槐林恢复过程中土壤氮素与微生物活性的变化

为了探讨不同生长年限的人工刺槐(Robinnia pseudoacacia)林对土壤中氮素组成与微生物活性的影响及机理,本文采用"时空互代"法进行野外选点调查和采样,对典型黄土丘陵区陕西省安塞纸坊沟小流域不同林龄(10 a、15 a、30 a、38 a)人工刺槐林和撂荒地3个土层(0~10 cm、10~30 cm和30~60 cm)中的全氮、铵态氮、硝态氮、有机氮、微生物生物量碳和磷、基础呼吸及基本理化性质进行了研究。结果表明:人工刺槐林地土壤微生物生物量碳、磷含量和微生物熵都显著高于撂荒地(P<0.05)。随着人工刺槐林生长年限的增加,各层土壤铵态氮、硝态氮和有机氮含量均逐渐增加,其中有机氮的增加最显著;土壤微生物生物量碳、磷含量显著增加;微生物熵显著增大而呼吸熵显著减小;土壤有机碳、速效磷含量总体上显著增加(P<0.05);容重和碳氮比则呈下降趋势。随着土层深度的增加,氮素、有机碳、速效磷和微生物生物量碳、磷含量显著减小(P<0.05);容重和pH显著增加。土壤微生物生物量碳、磷和呼吸熵均与有机氮、全氮、硝态氮显著正相关(P<0.05)。分析发现,刺槐的生长促使土壤中微生物可利用碳增加,提高了碳的利用率,使土壤微生物量碳、磷含量增加;微生物活性的提高反过来促进了土壤氮素含量的提高,土壤中有机氮含量显著增加。与10 a生刺槐林相比,30 a生林地土壤表层的全氮含量明显增加,氮素肥力由7级(0.40 g.kg 1)上升为5级(0.87 g.kg 1)水平。     

Ecotoxicological effects of fluorine deposits on microbial biomass and enzyme activity in grassland

The influence of atmospheric fluoride deposits on the soil microbial biomass and its enzyme activities was investigated near the aluminium smelter at Ranshofen, Upper Austria. Soil samples at various distances from the emission source were analyzed for water-extractable F and microbial activity. The water-extractable F contents at the sites examined reflected the gradient of F exposure (10 to 189 mg F kg^?l dry soil). The microbial activities increased with distance from the emission source and were inversely correlated with the degree of F contamination. The linear correlation coefficients between the water-extractable F concentrations and the microbial biomass, dehydrogenase and arylsulphatase activity were r=?0·8, ?0·86 and ?0·84, respectively. In the most contaminated soil (up to 189 mg F kg^?1), the microbial activities were only 5–20% of those in the unpolluted soil. The microbial biomass and dehydrogenase activity decreased substantially where the concentration of F exceeded 100mgkg^?1, whereas arylsulphatase activity was already inhibited at 20 mg kg^?1. The accumulation of organic matter near the smelter (123 mg F kg^?1) also indicated severe inhibition of the microbial activity by F. Our investigations show that the ratio of arylsulphatase to microbial biomass can be used as a sensitive index for evaluating environmental stress such as F Contamination.     

Erodibility in relation to water‐dispersible clay for some soils of eastern Nigeria

Water dispersible clay (WDC) can influence soil erosion by water. Therefore, in highly erodible soils such as the ones in eastern Nigeria, there is a need to monitor the clay dispersion characteristics to direct and modify soil conservation strategies. Twenty‐five soil samples (0–20 cm in depth) varying in texture, chemical properties and mineralogy were collected from various locations in central eastern Nigeria. The objective was to determine the WDC of the soils and relate this to selected soil physical and chemical attributes. The soils were analysed for their total clay (TC), water‐dispersible clay (WDC), clay dispersion ratio (CDR), dispersion ratio (DR), dithionite extractable iron (Fed), soil organic matter (SOM), exchangeable cations, exhangeable sodium percentage (ESP) and sodium adsorption ratio (SAR). Total clay contents of the soil varied from 80–560 g kg⁻¹. The USLE erodibility K ranges from 0·02 to 0·1 Mg h MJ⁻¹ mm and WEPP K fall between 1·2 × 10⁻⁶–1·7 × 10⁻⁶ kg s m⁻⁴. The RUSLE erodibility K correlated significantly with CDR and DR (r = 0·44; 0·39). Also, a positive significant correlation (r = 0·71) existed between WEPP K and RUSLE K. Soils with high clay dispersion ratio (CDR) are highly erodibile and positively correlates (p < 0·51) with Fed, CEC and SOM. Also, DR positively correlates with Mg²⁺ and SOM and negatively correlate with ESP and SAR. Principal component analysis showed that SAR, Na⁺ and percent base saturation play significant role in the clay dispersion of these soils. The implication of this result is that these elements may pose potential problem to these soils if not properly managed. Copyright © 2005 John Wiley & Sons, Ltd.     

CHANGES IN SOIL ORGANIC CARBON UNDER EUCALYPTUS PLANTATIONS IN BRAZIL: A COMPARATIVE ANALYSIS

Proper assessment of environmental quality or degradation requires knowledge of how terrestrial C pools respond to land use change. Forest plantations offer a considerable potential to sequester C in aboveground biomass. However, their impact on initial levels of soil organic carbon (SOC) varies from strong losses to gains, possibly affecting C balances in afforestation or reforestation initiatives. We compiled paired‐plot studies on how SOC stocks under native vegetation change after planting fast‐growth Eucalyptus species in Brazil, where these plantations are becoming increasingly important. SOC changes for the 0–20 and 0–40 cm depths varied between −25 and 42 Mg ha⁻¹, following a normal distribution centered near zero. After replacing native vegetation by Eucalyptus plantations, mean SOC changes were −1·5 and 0·3 Mg ha⁻¹ for the 0–20 and 0–40 cm depths, respectively. These are very low figures in comparison to C stocks usually sequestered in aboveground biomass and were statistically nonsignificant as demonstrated by a t‐test at p < 0·05. Similar low, nonsignificant SOC changes were estimated after data were stratified into first or second rotation cycles, soil texture and biome (savanna, rainforest or grassland). Although strong SOC losses or gains effectively occurred in some cases, their underpinning causes could not be generally identified in the present work and must be ascribed in a case basis, considering the full set of environmental and management conditions. We conclude that Eucalyptus spp. plantations in average have no net effect on SOC stocks in Brazil. Copyright © 2012 John Wiley & Sons, Ltd.     

土壤微生物对施入肥料氮的固持及其动态研究

采集长期定位试验（１４年）土壤（棕壤）进行盆栽试验,并应用同位素＾１５Ｎ示踪技术研究了土壤中微生物对肥料氮的固持及其动态,结果表明,施肥后５天土壤微生物对施入人肥氮的固持达达到最高,除单施氮肥处理的固持量占施入人肥氮量的５．４％外,其余各处理均天１３．３％－１５．４％间,施肥后土壤微生物量氮的增加主要来自化肥氮,后者占微生物体总氮量的６４．１％－８７．３％,在作物生长期间微生物固持的化肥氮逐渐释入     

灌溉和施氮对河西走廊紫花苜蓿生物量分配与 水分利用效率的影响

确定河西地区紫花苜蓿栽培草地的合理施氮量和灌溉量,对优化当地紫花苜蓿栽培草地生物量分配和提高水分利用效率具有重要意义。本研究利用田间试验研究了不同灌溉量(W1:当地灌溉量的60%;W2:当地灌溉量的80%;W3:当地灌溉量1 920 m3·hm-2)和施氮量[N1:0 kg(N)·hm-2;N2:40 kg(N)·hm-2;N3:80 kg(N)·hm-2;N4:120 kg(N)·hm-2]对2年生紫花苜蓿生物量分配特征及水分利用效率的影响。结果表明:灌溉量为W2和W3时均显著增加了紫花苜蓿株高、单株分枝数、地上生物量,及20~40 cm、40~60 cm和0~60 cm土层的根系体积、根系生物量和水分利用效率,且W2和W3的紫花苜蓿株高、单株分枝数和地上生物量差异不明显,说明采用当地灌溉量的80%水量时,紫花苜蓿水分利用效率最高。随着施氮量增加,紫花苜蓿单株分枝数、叶茎比、根系体积、根系生物量、地上和地下生物量比和水分利用效率均呈现先增加后降低的趋势,且在施氮量为80 kg(N)·hm-2时最大,说明紫花苜蓿根系发育和水分利用效率对氮的响应均存在剂量效应。在水氮互作条件下,处理W2N2或W2N3中紫花苜蓿株高、单株分枝数、根系体积和0~20 cm、20~40 cm、0~60 cm根系生物量及地上生物量与地下生物量比值和水分利用效率达到最优。结合上述分析得出在灌溉量W2和施氮N3时,紫花苜蓿地上地下生物量比值和水分利用效率达最大值,表明河西走廊紫花苜蓿栽培草地的适宜灌溉量为当地灌溉的80%,施氮量为80 kg·hm-2,此时紫花苜蓿水分利用效率和地上地下生物量比值配置最优。