We introduce a theoretical framework that predicts the optimum planting density and maximal yield for an annual crop plant. Two critical parameters determine the trajectory of plant growth and the optimal density, N opt , where canopies of growing plants just come into contact, and competition: (i) maximal size at maturity, M max , which differs among varieties due to artificial selection for different usable products; and (ii) intrinsic growth rate, g, which may vary with variety and environmental conditions. The model predicts (i) when planting density is less than N opt , all plants of a crop mature at the same maximal size, M max , and biomass yield per area increases linearly with density; and (ii) when planting density is greater than N opt , size at maturity and yield decrease with −4/3 and −1/3 powers of density, respectively. Field data from China show that most annual crops, regardless of variety and life form, exhibit similar scaling relations, with maximal size at maturity, M max , accounting for most of the variation in optimal density, maximal yield, and energy use per area. Crops provide elegantly simple empirical model systems to study basic processes that determine the performance of plants in agricultural and less managed ecosystems. E fficiency of agriculture will need to increase to feed the growing human population as arable land, water, and fertilizers become increasingly limited (1, 2). A relevant question is, What is the optimal density to plant seeds of an annual crop? The answer should be of interest to applied plant scientists who want to predict planting densities that maximize yields and to basic plant scientists who want to better understand the fundamental processes of growth and competition.Here we develop and test analytical models that predict the optimal seeding density that maximizes yield for annual crop plants. These models were inspired by theories and data on plant scaling relations (3-10). We modify the theories to model the growth and maturation of annual crops as a function of density and mature plant size. We evaluate the models using data from agricultural crops in controlled experiments in China. Empirical and Conceptual BackgroundThere is an intermediate seeding density for an annual crop that maximizes yield at harvest. When seeds are planted at lower density, yields are reduced because the plants grow to mature size without using all available resources. When seeds are planted at higher density, plants compete for resources and mature at smaller sizes; total yield declines because mature size per individual decreases faster than number of individuals per area increases.The dynamics of crop production can be modeled as the outcome of four interacting processes. First, the growth of an individual annual plant from germination to maturity traces a sigmoidal trajectory that reflects allocation of energy and biomass to new tissue as a function of plant size. Second, size at maturity depends on density: Initially all plants grow at nearmaximal rates, but if individuals ...

Land uses and cultivation are important factors controlling SOC storage on the Loess Plateau. These factors may also affect the relative importance of different mechanisms for the stabilization of organic matter in the soil. Easily oxidizable organic carbon (EOC), aggregation and aggregate C fractions in the soil were measured under different land uses. Aggregates were fractionated using a wet-sieving procedure to obtain the distribution of water-stable aggregates. The fractions of aggregates, aggregate SOC and aggregate EOC in grassland and forestland were generally higher than those in farmland. Furthermore, because conventional cultivation destroyed aggregates, the dominant aggregate size fractions were b 0.5 mm for farmland and N0.5 mm for other land uses. Compared to the corresponding values in farmland, the mean weight diameter (MWD) in forestland and grassland increased by 808%-417%, and the stability ratio of water-stable aggregate (WSAR) increased by 920%-553%. Aggregate formation and its dominant size fraction were associated closely with its carbon fractions. SOC and EOC in farmland tended to be concentrated in smaller-sized aggregates, whereas SOC and EOC under other land uses tended to concentrate in larger-sized aggregates. EOC tended to concentrate in larger aggregates than SOC. The small fractions of the aggregates formed large fractions by combining with fresh organic matter. So converting slope farmland to forestland and grassland could improve the storage and quality of SOC, and the tendency of SOC transfer.

Fertilization is important to increase crop yields, but long-term application of fertilizers probably aggravated the risk of heavy metals in acidic soils. In this study, the effect of 22-year fertilization and cropping on accumulation, availability, and uptake of heavy metals in red soil was investigated. The results showed that pig manure promoted significantly cadmium (Cd) accumulation (average 1.1 mg kg(-1)), nearly three times higher than national soil standards and, thus, increased metal availability. But the enrichment of heavy metals decreased remarkably by 50.5 % under manure fertilization, compared with CK (control without fertilization). On the contrary, chemical fertilizers increased greatly lead (Pb) availability and Cd activity; in particular, exceeding 85 % of soil Cd became available to plant under N (nitrogen) treatment during 9-16 years of fertilization, which correspondingly increased their enrichment by 29.5 %. Long-term application of chemical fertilizers caused soil acidification and manure fertilization led to the increase in soil pH, soil organic matter (SOM), and available phosphorus (Olsen P), which influenced strongly metal behavior in red soil, and their effect had extended to deeper soil layer (20∼40 cm). It is advisable to increase application of manure alone with low content of heavy metals or in combination with chemical fertilizers to acidic soils in order to reduce toxic metal risk.

The present regression models in digital soil mapping usually assume that relationships between soil properties and environmental variables are always fixed (as in MLR) or varying (as in GWR) in geographical space. In reality, some of the environmental variables may be fixed in affecting soil property variation and some are local varying. In this study, a mixed geographically weighted regression (MGWR) method which can deal with fixed and varying spatial relationships between a target variable and its environmental variables were proposed and used to predict topsoil soil organic matter (SOM) concentration in two study areas (Heshan, Heilongjiang province and Xuancheng, Anhui province, China) at two scales. Three groups of sample sets were created based on the total samples in the study areas to evaluate the robustness and stability of the model. Multiple linear regression (MLR), geographically weighted regression (GWR), GWR-kriging (GWRK), local regression-kriging (LRK), kriging with an external drift (KED), and ordinary kriging (OK) were used for comparison with MGWR. The validation results showed that the use of MGWR reduced the RMSE of GWR by 10.5% and 7.6% on average, reduced the RMSE of MLR by 12.8% and 9.9% on average for Heshan and Xuancheng study areas respectively. MGWR also showed a good competitiveness when compared with GWRK, LRK, KED and OK. In Heshan study area, the influence of flow length, relative position index, foot slope and distance to the nearest drainage were constant, whereas the elevation, topographic wetness index and valley index showed different influence in different regions. In Xuancheng study area, the fixed environmental variables were profile curvature, topographic wetness index and slope, whereas the varying environmental variables were precipitation, temperature, elevation, and limestone. The results indicate that the accuracy of predictions can be improved by adaptive coefficient according to the variation of environmental variables as implemented in MGWR compared with others considering only the local or global relationships. It was concluded that mixed geographically weighted regression model could be a potential method for digital soil mapping.

A factorial study was conducted to evaluate the phytotoxicity of copper oxide nanoparticles (nCuO, 0.1–100 mg/L), arsenic (As, 0 and 10 mg/kg), and their interaction to rice plants (Oryza sativa japonica ‘Koshihikari’) during the life cycle. No significant effect was observed on seed germination. The main effects of nCuO and As were observed on lengths and biomasses of seedling shoots and roots and on root branching. The interaction between nCuO and As also significantly influenced these parameters. nCuO addition increased Cu uptake in seedlings and generally improved seedling growth. With As addition, As was highly concentrated in roots and increased in shoots, and seedling growth was also inhibited. Additionally, nCuO and As had significant main and interaction effects on mature plant dry biomass, panicle number, total grain weight, average grain weight, and several other panicle parameters. Moreover, nCuO and As interacted to affect panicle emergence. nCuO also decreased As accumulation in dehusked grains. The accelerated heading stage by nCuO may help shorten the life cycle of rice plants, thereby reducing As accumulation in grains. This study is the first to examine the influence of nCuO in combination with As on the life cycle of rice plants.

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The performance of hot-water extraction (HWE) and steam treatment (ST), followed by kraft pulping were compared for production of high puritygrade dissolving pulp from green bamboo. With the same prehydrolysis intensity (represented by the P-factor), the fractionation efficiency of HWE is far lower than that of ST. Because of lower removal of non-cellulosic components, the solid residue from HWE (even at approximately double the prehydrolysis intensity, P-factor = 1,379) required more active alkali (AA) during kraft pulping to obtain a cellulose purity equivalent to that achieved by the ST (Pfactor = 756)-kraft process. To reach equivalent hemicellulose removal, HWE required more severe intensity than ST. However, FTIR and SEM characterizations of solid residue confirmed that intensified HWE resulted in significant lignin condensation. Antagonistic effects of hemicellulose removal and lignin condensation extent on subsequent kraft pulping were therefore more apparent in HWE than that in ST. Under the same kraft pulping conditions, lignin condensation from a severely intensified HWE process (P-factor = 2,020) caused greater cellulose yield and viscosity loss than that found for ST. Finally, at a given residual pentosan or lignin content, the cellulose yields from all HWE-kraft pulps were about 3 % lower than those from ST-kraft pulps. Consequently, based on an optimally setup chlorine dioxide bleaching stage, a cellulosic pulp with alpha-cellulose content of 97.6 % and viscosity of 927 mL/g was successfully produced from a ST-kraft pulp (P-factor = 756, AA = 19 %).