腾格里沙漠东南缘沙质草地灌丛化对地表径流及氮流失的影响

以腾格里沙漠东南缘沙质草地和灌丛生境为研究对象,采用人工模拟降雨实验对草地样方、灌丛间裸地样方及含灌丛斑块样方的产流及氮流失过程进行观测,揭示了地表径流及其引起的氮流失对沙质草地灌丛化的响应.结果表明:(1)草地样方出现表面积水和地表径流的时间及开始产流需要的降雨量均大于含灌丛斑块样方和灌丛间裸地样方,裸地样方最小;灌丛生境径流系数为34.46％,显著小于裸地样方,大于含灌丛样方,产流量是沙质草地生境的2.26倍;表明灌木入侵造成的植被盖度下降引起了土壤水分入渗率的减小和地表产流的增加.(2)含灌丛样方单位体积径流含氮量瞬时值大于裸地样方,小于草地样方,三类样方瞬时值与单位时间径流量均呈线性负相关;草地样方单位时间氮流失量略小于含灌丛样方,两者均显著小于灌丛间裸地样方;灌丛生境氮流失总量为0.23 g/m2,是草地生境的2.09倍,灌丛和草地生境单位体积径流含氮量总体平均值分别为0.011 g/L、0.012 g/L;表明沙质草地灌丛化引起了养分流失的显著增加.     

Nutrient losses in runoff from grasslandand shrubland habitats in southern New Mexico: II. Field plots

Losses of dissolved nutrients (N, P, K, Ca, Mg, Na,Cl, and SO₄) in runoff were measured on grasslandand shrubland plots in the Chihuahuan desert ofsouthern New Mexico. Runoff began at a lowerthreshold of rainfall in shrublands than ingrasslands, and the runoff coefficient averaged 18.6%in shrubland plots over a 7-year period. In contrast,grassland plots lost 5.0 to 6.3% of incidentprecipitation in runoff during a 5.5-year period. Nutrient losses from shrubland plots were greater thanfrom grassland plots, with nitrogen losses averaging0.33 kg ha^–1 yr^–1 vs0.15 kg ha^–1 yr^–1, respectively, during a 3-year period. Thegreater nutrient losses in shrublands were due tohigher runoff, rather than higher nutrientconcentrations in runoff. In spite of these nutrientlosses in runoff, all plots showed net accumulationsof most elements due to inputs from atmosphericdeposition. Therefore, loss of soil nutrients byhillslope runoff cannot, by itself, account for thedepletion of soil fertility associated withdesertification in the Chihuahuan desert.     

Implications of land-cover types for soil erosion on semiarid mountain slopes: Towards sustainable land use in problematic landscapes

The impact of land-cover types on soil erosion and runoff, as well as on physico-chemical soil properties, was monitored. The study area, an agroforestry landscape was located in Sierra Nevada Mountains in south-eastern Spain. Eight land-cover types were investigated: farmland planted with olive, almond, and cereals; forest with P. halepensis and P. sylvestris; shrubland; grassland; and abandoned farmland. The erosion plots replicated twice were located on hillslopes, where erosion and runoff were measured after 22 storm events. Forest dominated by Pinus stands exhibited significantly the lowest amounts of erosion and runoff, contrasting with abandoned farmland. Olive had higher erosion than did almond, cereals, or grasslands, but with the highest runoff rate under almond groves. The erosion and runoff response to shrubland showed an intermediate situation between forest and farmland–grassland uses. Under forest and shrubland, better soil properties were determined, especially higher organic C and total N, and lower soil-bulk density. Erosion was highly dependent on runoff, bulk density, soil organic C, and the degree of soil surface covered. Thus, the alteration in land cover is essential to an understanding of productivity of soil undergoing erosion, as sustainable planning can mitigate soil-degradation processes in the overall agroforestry landscape.     

水网平原地区不同种植类型农田氮磷流失特征

采用田间径流小区定位研究方法,在浙江省绍兴县选择27块农田,研究了自然降雨条件下水网平原地区7种种植类型农田N、P的径流流失特征、负荷及影响因素.结果表明： 农田径流总P（TP）、水溶态P（DP）和颗粒态P（PP）的年流失量平均分别为4.75、0.74和4.01 kg·hm^-2;PP占TP的比例高于DP.径流总N（TN）、水溶态总N（DTN）、水溶态有机N（DON）、NH₄⁺-N和NO₃^--N的年流失量平均分别为21.87、17.19、0.61、3.63和12.95 kg·hm^-2;流失的DTN各组分以NO₃^--N为主,其次为NH₄⁺-N,DON的比例较低.不同种植类型农田径流TN、DTN、DON和NO₃^--N的流失量由低至高依次为：休闲地<苗木地<单季晚稻农田<双季稻农田<油菜(或小麦)-单季水稻农田<小麦-早稻-晚稻农田<蔬菜地,而径流TP和PP的流失量依次为：休闲地<苗木地<单季晚稻、双季稻农田<小麦-早稻-晚稻农田<油菜(或小麦)-单季水稻农田<蔬菜地,不同种植类型间的DP流失量差异较小.N、P流失主要发生在作物生产期间,TN和TP的流失比例随作物复种指数的提高而增加.TN、DTN和NO₃^--N流失量主要与N肥施用量有关,土壤中NO₃^--N含量对TN和DTN流失量也有明显影响;农田DON的流失除与N肥施用量有关外,还受土壤全N和有机质积累的影响;NH₄⁺-N的流失量主要与土壤NH₄⁺-N水平有关,受N肥施用量的影响不明显;径流TP和PP的流失量受P肥施用量、土壤P积累的共同影响,而DP的流失与施P量关系不大,但与土壤全P和有效P都存在显著相关关系.     

宁夏东部荒漠草原-灌丛地转变过程土壤氮素响应

选取近30年荒漠草原灌丛引入形成的典型草地-灌丛镶嵌体内部的荒漠草地、草地边缘、灌丛边缘、灌丛地为研究样地,对各样地及其微生境(植丛与空斑)相关土壤指标进行测定,以了解荒漠草地向灌丛地转变过程中土壤氮素的响应特征.结果 表明:随草地灌丛化转变,草本与灌丛生物量均增加,其中一年生草本随灌丛引入增加明显;土壤水分、全碳、全...     

基于黄土高原关键带类型的土地利用与年径流产沙关系空间分异研究

区域植被恢复改变了土地利用类型,从而有效控制了水土流失,但土地利用与水土流失关系的空间分异尚未明晰。整合了黄土高原坡面径流小区试验观测研究文献59篇和1121条年径流产沙记录,以8大关键带类型作为空间分层依据,采用地理探测器分析了土地利用与年径流产沙关系的空间分异。结果显示：撂荒地的年均径流量和产沙量最高分别为35.99 mm和4208.82 g/m²,撂荒地、裸地和耕地的产流产沙能力显著高于人工草地、林地、自然草地和灌丛,灌丛和林地的年均产沙量显著低于人工和自然草地（P<0.05）;除了撂荒地的年均产沙量在山地森林关键带最高（16240.40 g/m²）外,在丘陵沟壑农林草交错关键带的撂荒地年均径流产沙显著高于丘陵农业-草地关键带,丘陵沟壑农林草交错关键带和丘陵农业-草地关键带裸地、耕地的产流产沙能力较高,人工草地和灌丛年均产沙量显著高于其他关键带类型（P<0.05）;在山地森林关键带的林地年均径流量、径流系数和产沙量最低,分别为1.56 mm、0.41%和307.36 g/m²,而自然草地在各关键带类型都有较高的年均产流量和较低的年均产沙量;坡面径流小区的局地特征（如土地利用、面积、坡度、坡长）是影响年径流产沙关键带分异的首要因素,且存在多因子互作、非线性增强的关系。这些结果表明植被恢复能有效地保持水土,但是区域植被恢复时需要选择合适的类型,黄土丘陵沟壑区应首选自然草地、灌丛和林地。研究可为黄土高原区域植被恢复的优化配置提供科学依据。     

Responses to fertility and disturbance in a low-diversity grassland

We examined variation in species composition in a low-diversity, anthropogenic grassland in response to 11 years of nitrogen (N) manipulation and disturbance. The species-poor grassland (2–3 species/0.5 m²) represents a wide spread vegetation type (>10 million ha in North America) dominated by the introduced perennial grasses Bromus inermis and Agropyron cristatum. Four levels of N and three of soil disturbance were applied in all combinations to plots (5 × 15 m, N = 120) in a completely randomized design each year. Seeds or transplants of 47 species were added to ensure that dispersal was not a barrier to changes in species composition. After 11 years of treatment, all but the most disturbed plots continued to be dominated by B. inermis. The cover of the second-most abundant species, A. cristatum, decreased with disturbance but did not vary significantly with N. Despite the lack of changes in the identity of the dominant species, our environmental manipulations strongly influenced ecosystem characteristics. Added N increased soil available N, and decreased the cover of bare ground and light availability. Soil disturbance decreased aboveground biomass, and increased the cover of bare ground and light availability. Sawdust application, designed to decrease N availability, significantly reduced community biomass, and increased light availability and the cover of bare ground, but did not alter nutrient availability or species composition. The results highlight the difficulty of restoring diversity in species-poor, anthropogenic communities dominated by introduced species, and thus the importance of conserving remnants of diverse natural grasslands.     

Effects of an African grass invasion on Hawaiian shrubland nitrogen biogeochemistry

African perennial C₄ grasses are highly successful invaders in Hawaiian ecosystems. We examined the effects of African molasses grass (Melinis minutiflora Beauv.) on Hawaiian shrubland nitrogen (N) dynamics without the influence of fire disturbance. Vegetation tissue carbon and nitrogen chemistry, soil inorganic N pools, net N mineralization rates, and total soil N were studied in three adjacent areas: a monospecificMelinis grassland, a mixed grass/shrubland mosaic, and an un-invaded shrubland.Melinis plots within the mosaic area exhibited the largest inorganic N pools and fastest net N mineralization rates, but were temporally variable with grass phenology. Un-invaded shrubland plots contained the smallest inorganic N pools and lowest net N mineralization rates. Grass foliar C:N and litter C:N were lower than those of common shrubland species, providing one possible link between species and ecosystem N dynamics at this site. The combined effects of N cycle modification, successful light competition, and fire-cycle enhancement make the invasion ofMelinis a significant perturbation to Hawaiian shrubland ecosystem function and successional dynamics. ei]Section editor H Lambers     

Impacts of Biological Soil Crust Disturbance and Composition on C and N Loss from Water Erosion

In this study, we conducted rainfall simulation experiments in a cool desert ecosystem to examine the role of biological soil crust disturbance and composition on dissolved and sediment C and N losses. We compared runoff and sediment C and N losses from intact late-successional dark cyanolichen crusts (intact) to both trampled dark crusts (trampled) and dark crusts where the top 1 cm of the soil surface was removed (scraped). In a second experiment, we compared C and N losses in runoff and sediments in early-successional light cyanobacterial crusts (light) to that of intact late-successional dark cyanolichen crusts (dark). A relatively high rainfall intensity of approximately 38 mm per 10-min period was used to ensure that at least some runoff was generated from all plots. Losses of dissolved organic carbon (DOC), dissolved organic nitrogen (DON), and ammonium (NH₄⁺ ) were significantly higher from trampled plots as compared to scraped and intact plots. Sediment C and N losses, which made up more than 98% of total nutrient losses in all treatments, were more than 4-fold higher from trampled plots relative to intact plots (sediment C g/m², intact = 0.74, trampled = 3.47; sediment N g/m², intact = 0.06, trampled = 0.28). In light crusts, DOC loss was higher relative to dark crusts, but no differences were observed in dissolved N. Higher sediment loss in light crusts relative to dark crusts resulted in 5-fold higher loss of sediment-bound C and N. Total C flux (sediment + dissolved) was on the order of 0.9 and 7.9 g/m² for dark and light crusts, respectively. Sediment N concentration in the first minutes after runoff from light crusts was 3-fold higher than the percent N of the top 1 cm of soil, suggesting that even short-term runoff events may have a high potential for N loss due to the movement of sediments highly enriched in N. Total N loss from dark crusts was an order of magnitude lower than light crusts (dark = 0.06 g N/m², light = 0.63 g/m²). Overall, our results from the small plot scale (0.5 m²) suggest that C and N losses are much lower from intact late-successional cyanolichen crusts as compared to recently disturbed or early-successional light cyanobacterial crusts.     

Vegetation dynamics in a Quercus‐Juniperus savanna: An isotopic assessment

Abstract. Woody plants are increasing in many grassland and savanna ecosystems around the world. As a case in point, the Edwards Plateau of Texas, USA, is a vast region (93 000 km²) in which rapid woody encroachment appears to be occurring. The native vegetation (prior to the Anglo‐European settlement 150–200 yr ago) and the biogeochemical consequences of woody encroachment in this region, however, are poorly understood. To assess these matters we measured plant and soil δ¹³C, soil organic C and soil N content from grasslands and two important woody patch types (mature Quercus virginiana clusters and Juniperus ashei woodlands) in this region. Soil δ¹³C values showed that relative productivity of C₃ species has increased in grassland and both woody habitats in recent times. δ¹³C of SOC in grasslands and Q. virginiana clusters increased with depth from the litter layer to 30 cm (grasslands =?21 to ?13‰Q. virginiana clusters =?27 to ?17‰) and were significantly different between habitats at all depths, indicating that Q. virginiana has been a long‐term component of the landscape. In J. ashei woodlands, soil δ¹³C values (at 20–30 cm depth) near the woodland edge (‐13‰) converged with those of an adjacent grassland (‐13‰) while those from the woodland interior (‐15‰) remained distinct, indicating that the woodland has been present for many years but has recently expanded. Concentrations and densities of SOC and total N were generally greater in woody patches than in grasslands. However, differences in the amount of SOC and N stored beneath the two woody patch types indicates that C and N sequestration potentials are species dependent.     

The biogeochemistry of a north-temperate grassland with native ungulates: Nitrogen dynamics in Yellowstone National Park

Nutrient dynamics of large grassland ecosystems possessing abundant migratory grazers are poorly understood. We examined N cycling on the northern winter range of Yellowstone National Park, home for large herds of free-roaming elk (Cervus elaphus) and bison (Bison bison). Plant and soil N, net N mineralization, and the deposition of ungulate fecal-N were measured at five sites, a ridgetop, mid-slope bench, steep slope, valley-bottom bench, and riparian area, within a watershed from May, 1991 to April, 1992.Results indicated similarities between biogeochemical properties of Yellowstone grassland and other grassland ecosystems: (1) landscape position and soil water affected nutrient dynamics, (2) annual mineralization was positively related to soil N content, and (3) the proportion of soil N mineralized during the year was negatively related to soil C/N.Grazers were a particularly important component of the N budget of this grassland. Estimated rates of N flow from ungulates to the soil ranged from 8.1 to 45.6 kg/ha/yr at the sites (average = 27.0 kg/ha/yr), approximately 4.5 times the amount of N in senescent plants. Rates of nitrogen mineralization for Yellowstone northern range grassland were higher than those measured in other temperate grassland ecosystems, possibly due to grazers promoting N cycling in Yellowstone.     

Changes in Soil Carbon and Nitrogen following Land Abandonment of Farmland on the Loess Plateau,China

The revegetation of abandoned farmland significantly influences soil organic C (SOC) and total N (TN). However, the dynamics of both soil OC and N storage following the abandonment of farmland are not well understood. To learn more about soil C and N storages dynamics 30 years after the conversion of farmland to grassland, we measured SOC and TN content in paired grassland and farmland sites in the Zhifanggou watershed on the Loess Plateau, China. The grassland sites were established on farmland abandoned for 1, 7, 13, 20, and 30 years. Top soil OC and TN were higher in older grassland, especially in the 0–5 cm soil depths; deeper soil OC and TN was lower in younger grasslands (<20 yr), and higher in older grasslands (30 yr). Soil OC and N storage (0–100 cm) was significantly lower in the younger grasslands (<20 yr), had increased in the older grasslands (30 yr), and at 30 years SOC had increased to pre-abandonment levels. For a thirty year period following abandonment the soil C/N value remained at 10. Our results indicate that soil C and TN were significantly and positively correlated, indicating that studies on the storage of soil OC and TN needs to focus on deeper soil and not be restricted to the uppermost (0–30 cm) soil levels.     

Seed bank dynamics of the desert cactus Opuntia rastrera in two habitats from the Chihuahuan Desert

In desert environments the main input to the seed bank of many succulents is the seed rain through zoochory while high levels of granivory by rodents, birds and ants are the main cause of subsequent losses. In the patchy environment of arid lands the characteristics of both processes may vary between habitats causing differences in the recruitment of new genets. To test this hypothesis we used populations of the desert cactus Opuntia rastrera which has different recruitment rates in the two adjacent habitats where it grows. In Opuntia-dominated scrublands (nopaleras, density ca. 4,000 plants/ha) 1 seedling out of 7,000,000 seeds establish, whereas in grasslands (density ca. 100 plants/ha) this ratio is 1:20,000. From 1996 until 1998 the seed rain, seed removal by granivores and seed abundance in the soil were monitored in both habitats. Results showed striking differences in the dynamics of the seed bank of both habitats. Seed rain was 8.5 times bigger in nopaleras than in grasslands. In nopaleras most seeds were removed by rodents while the quantities of seeds removed by rodents, birds and ants in grasslands were similar. One year after dispersal (the time necessary to break seed dormancy) only 6% of original nopalera seeds and 12% of grassland seeds remained. After germination trials only 1% (ca. 15,000 seeds/ha) and 2% (ca. 2,500 seeds/ha) respectively were viable. These differences in the effective seed bank (6 times bigger in nopaleras) can not explain the differences in genet recruitment (which is several orders of magnitude bigger in grasslands). Apparently the between habitat difference in nurse plant availability and in rodent density (which inflict a strong hervibory upon seedlings) can explain the differences in genet recruitment. It is speculated that this between habitat difference in genet recruitment suggests that the species evolved in less extreme environments (e.g. grasslands) than desert scrublands which, in turn, are colonised due to the singular ability of O. rastrera for vegetative propagation. This revised version was published online in August 2006 with corrections to the Cover Date.     

Measured and simulated nitrous oxide emissions from ryegrass- and ryegrass/white clover-based grasslands in a moist temperate climate

There is uncertainty about the potential reduction of soil nitrous oxide (N(2)O) emission when fertilizer nitrogen (FN) is partially or completely replaced by biological N fixation (BNF) in temperate grassland. The objectives of this study were to 1) investigate the changes in N(2)O emissions when BNF is used to replace FN in permanent grassland, and 2) evaluate the applicability of the process-based model DNDC to simulate N(2)O emissions from Irish grasslands. Three grazing treatments were: (i) ryegrass (Lolium perenne) grasslands receiving 226 kg FN ha(-1) yr(-1) (GG+FN), (ii) ryegrass/white clover (Trifolium repens) grasslands receiving 58 kg FN ha(-1) yr(-1) (GWC+FN) applied in spring, and (iii) ryegrass/white clover grasslands receiving no FN (GWC-FN). Two background treatments, un-grazed swards with ryegrass only (G-B) or ryegrass/white clover (WC-B), did not receive slurry or FN and the herbage was harvested by mowing. There was no significant difference in annual N(2)O emissions between G-B (2.38±0.12 kg N ha(-1) yr(-1) (mean±SE)) and WC-B (2.45±0.85 kg N ha(-1) yr(-1)), indicating that N(2)O emission due to BNF itself and clover residual decomposition from permanent ryegrass/clover grassland was negligible. N(2)O emissions were 7.82±1.67, 6.35±1.14 and 6.54±1.70 kg N ha(-1) yr(-1), respectively, from GG+FN, GWC+FN and GWC-FN. N(2)O fluxes simulated by DNDC agreed well with the measured values with significant correlation between simulated and measured daily fluxes for the three grazing treatments, but the simulation did not agree very well for the background treatments. DNDC overestimated annual emission by 61% for GG+FN, and underestimated by 45% for GWC-FN, but simulated very well for GWC+FN. Both the measured and simulated results supported that there was a clear reduction of N(2)O emissions when FN was replaced by BNF.     

Changes in dry matter and nutrient yields during the restoration of species-rich grasslands

Abstract. In two moderately productive grasslands that had been withdrawn from agricultural use, one on humic sandy soil and one on heavy clay, the decrease in dry matter, N-, P- and K-yields were measured for 14 and 11 years respectively, after fertilization had been stopped. The grasslands were mown twice a year. The yields and concentrations of nutrients were compared with those of already unproductive grasslands under the same growing conditions. On sandy soil, the annual dry matter yield decreased from 10.2 to 6.5 ton / ha in four years. After nine years, the dry matter (4.1 ton / ha) and K (20.5 kg / ha) yields were as low as those of the unproductive grassland. At that time yields and concentrations of N and P were still greater. The low K- concentration of 0.50 % and the lack of response upon N fertilizing indicate that the supply of K limits biomass production. On clay, during the first three years, the dry matter yield decreased from 10.2 to 5.0 ton / ha, after 6 yr the production increased again. The low N-concentration of 1.52 % after 10 yr indicates that N is likely to limit biomass production, but the low P-concentration of 0.14% indicates that P shortage can also be expected. In the lower productive grassland on sand the number of species that invaded was higher.     

Effects of Revegetation on Soil Microbial Biomass,Enzyme Activities,and Nutrient Cycling on the Loess Plateau in China

Revegetation is a traditional practice widely used for soil and water conservation on the Loess Plateau in China. However, there has been a lack of reports on soil microbial–biochemical indices required for a comprehensive evaluation of the success of revegetation systems. In this study, we examined the effects of revegetation on major soil nutrients and microbial–biochemical properties in an artificial alfalfa grassland, an enclosed natural grassland, and an artificial shrubland (Caragana korshinskii), with an abandoned cropland as control. Results showed that at 0–5, 5–20, and 20–40 cm depths, soil organic carbon, alkaline extractable nitrogen and available potassium were higher in natural grassland and artificial shrubland compared with artificial grassland and abandoned cropland. Soil microbial biomass C (Cmic) and phosphorous (Pmic) substantially decreased with depth at all sites, and in abandoned cropland was significantly lower than those of natural grassland, artificial grassland, and artificial shrubland at the depth of 0–5 cm. Soil microbial biomass N (Nmic) was higher in artificial shrubland and abandoned cropland compared with that in natural and artificial grasslands. Both Cmic and Pmic were significantly different between the 23‐year‐old and the 13‐year‐old artificial shrublands at the 0–5 cm depth. The activities of soil invertase, urease, and alkaline phosphatase in natural grassland and artificial shrubland were higher than those in artificial grassland and abandoned cropland. This study demonstrated that the regeneration of both natural grassland and artificial shrubland effectively preserved and enhanced soil microbial biomass and major nutrient cycling, thus is an ecologically beneficial practice for recovery of degraded soils on the Loess Plateau.     

宁夏东部荒漠草原向灌丛地人为转变过程中土壤胞外酶活性响应

以宁夏东部荒漠草原-灌丛地典型镶嵌体内部荒漠草地、草地边缘、灌丛边缘、灌丛地为对象,对各样地植丛和空斑下土壤特性及6种土壤胞外酶活性(纤维二糖水解酶、β-1,4-木糖苷酶、β-1,4葡萄糖苷酶、β-1,4-乙酰基氨基葡萄糖苷酶、亮氨酸氨基肽酶和碱性磷酸酶)进行分析,研究荒漠草原向灌丛地人为转变过程中胞外酶的响应特征。结果表明: 荒漠草原向灌丛地转变过程中,土壤水分、有机碳、全氮、全磷、微生物生物量碳、微生物生物量氮均显著降低,且灌丛地显著低于草地26.0%～88.5%;除草地边缘土壤水分、有机碳空斑略高于植丛外,其他指标均表现为各样地植丛显著高于空斑3.9%～82.3%。6类土壤胞外酶活性在转变过程中均呈下降趋势,降幅为22.1%～82.4%,其中亮氨酸氨基肽酶和碱性磷酸酶降低最为显著,分别降低82.4%和75.5%;除灌丛地β-1,4-乙酰基氨基葡萄糖苷酶在空斑显著高于植丛外,其他胞外酶活性均表现为各样地植丛高于空斑10.7%～42.7%;转变过程中6类胞外酶活性之间呈显著正相关,且均与土壤特性呈不同程度正相关,其中各类土壤胞外酶活性对土壤微生物生物量碳、氮及全氮响应较为积极。     

Habitat preferences of the threatened Black-eared Wheatear Oenanthe hispanica in southern Italy

Capsule We analysed habitat preferences of Black-eared Wheatear Oenanthe hispanica of the eastern subspecies (O. hispanica melanoleuca) in South Italy, considering 1-ha (coinciding with territory size) square plots (49 occupied and 49 unoccupied). We used multi-adaptive regression splines to model habitat preferences. Black-eared Wheatear occurrence was positively associated with three factors: aspect (a SE orientation was preferred), cover of grazed grassland and cover of bare ground. Species conservation should be based on the maintenance of grazed grasslands, especially on SE-facing slopes, and including at least 2500 m² of bare ground per ha.     

Richness and composition of herbaceous species in restored shrubland and grassland ecosystems in the northern Loess Plateau of China

Restored grasslands and shrublands are integral parts of the semi-natural landscape and are of major importance for biodiversity in the northern Loess Plateau. Determining the underlying factors that control the richness and composition of herbaceous species in restored grasslands and shrublands is urgently needed. Thus, the specific objective of this study was to evaluate the relative importance of soil, plant, and topographic explanatory variables affecting the richness and composition of herbaceous species in restored shrubland and grassland ecosystems in a typical watershed within the northern Loess Plateau. In this study, 27 restored grassland sites and 16 restored shrubland sites were sampled during September 2009. Using variation partitioning (partial canonical correspondence analysis), we determined the individual and shared effects of these three sets of explanatory variables on herbaceous biodiversity in the two restored habitats. Most of the explained variation in plant diversity was related to the pure effect of soil, plant, and topographic variables. Restored shrublands had significantly more species than grasslands, and abandoned dam farmlands had significantly more species than other grassland sites. Moreover, botanical diversity responded differently to the explanatory variables in different plant communities. The pure effects of soil properties, soil moisture in particular, accounted for the largest fractions of explained variation in species diversity in restored grasslands. Both plant and topographic variables had balancing pure effects on species diversity in restored shrublands, in particular the shrub density and slope angle. We conclude that the maintenance of a moderate density of shrubs (less than 3600 shrubs per ha), construction of check-dams, and grazing at a low stocking rate, taking conditions of soil and topographic site into account, may help to conserve biodiversity in the northern Loess Plateau.     

Calcium, potassium, and magnesium cycling in aMexican tropical dry forest ecosystem

We estimated the fluxes, inputs and outputs of Ca, K,and Mg in a Mexican tropical dry forest. The studywas conducted in five contiguous small watersheds(12–28 ha) gauged for long-term ecosystem research. A total of five 80 × 30 m plots were used for thestudy. We quantified inputs from the atmosphere,dissolved and particulate-bound losses, throughfalland litterfall fluxes, and standing crop litter pools. Mean cation inputs for a six-year period were 3.03 kg/ha for Ca, 1.31 kg/ha for K, and 0.80 kg/ha for Mg. Mean outputs in runoff were 5.24, 2.83, and 1.79 kg/ha, respectively. Calcium, K, and Mgconcentrations increased as rainfall moved through thecanopy. Annual Ca return in the litterfall (11.4 g/m²) was much higher than K (2.3 g/m²)and Mg (1.6 g/m²). Litterfall represented 99%of the Ca, 84% of the Mg, and 53% of the K, totalaboveground return to the soil. Calcium concentrationin standing litter (3.87%) was much higher than K(0.38%) and Mg (0.37%). These concentrations werehigher (Ca), lower (K), or similar (Mg) to those inlitterfall. Residence times on the forest floor were0.86, 1.17, and 1.77 yr for K, Mg, and Carespectively. Compared to the residence time fororganic matter at the site (1.31 yr), these resultssuggest slow mineralization for Ca in this ecosystem. Budget estimates were calculated for a wet and a dryyear. Results indicated that nutrients accumulated inthe dry but that nutrients were lost during the wetyear. Comparison of Ca, K, and Mg losses in streamwater with the input rates from the atmosphere for thesix-year period show that inputs are lower thanoutputs in the Chamela tropical dry forestecosystem.