Soil organic carbon (SOC) is the most often reported attribute and is chosen as the most important indicator of soil quality and agricultural sustainability. In this review, we summarized how cultivation, crop rotation, residue and tillage management, fertilization and monoculture affect soil quality, soil organic matter (SOM) and carbon transformation. The results confirm that SOM is not only a source of carbon but also a sink for carbon sequestration. Cultivation and tillage can reduce soil SOC content and lead to soil deterioration. Tillage practices have a major effect on distribution of C and N, and the rates of organic matter decomposition and N mineralization. Proper adoption of crop rotation can increase or maintain the quantity and quality of soil organic matter, and improve soil chemical and physical properties. Adequate application of fertilizers combined with farmyard manure could increase soil nutrients, and SOC content. Manure or crop residue alone may not be adequate to maintain SOC levels. Crop types influence SOC and soil function in continuous monoculture systems. SOC can be best preserved by rotation with reduced tillage frequency and with additions of chemical fertilizers and manure. Knowledge and assessment of changes (positive or negative) in SOC status with time is still needed to evaluate the impact of different management practices.

Plants grown at noncompetitive densities (isolated plants) can be used to relate competitive pressure on yield and yield components at high plant densities. The main objective of this research was to quantify the sensitivity of grain yield and its components to manipulation of crowding stress in corn (Zea mays L.). The experiment was conducted in Deerfield, MA (1986, 1987, and 2000), and Shoush, Iran (1998 and 1999). Three single‐ear corn hybrids were planted at six densities (0.25, 3, 4.5, 6, 9, and 12 plants m−2), the lowest density being considered an isolated density. The higher three densities (6, 9, and 12 plants m−2) were combined with three removal treatments, consisting of removal of alternate plants in rows at different stages of growth. Intensity of competition was quantified by comparing grain yield and its components of plants in these densities with those of isolated plants. The highest grain yield in all experimental sites was obtained from 9 plants m−2 and for total biomass yield between 9 and 12 plants m−2. Kernel yield per plant decreased linearly in all hybrids as plant density intensified. All yield components had a linear decline in response to increased competition pressure. The reduction in kernel yield was attributed most to the reduction in number of kernels per row. Removal treatments indicated that early competition during vegetative growth had no or little effect on final grain yield. Plant competition between the vegetative stage and anthesis had a large effect on grain yield reduction, which ranged from 8 to 21% in different hybrids and experimental sites. Increased assimilate supply through plant removal again confirmed that adjustments in grain yield occurred primarily through kernel number per row.

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Seeds are the primary sinks for photosynthates during reproductive growth. Variation in light intercepted during and after seed initiation has been found a major environmental determinant of soybean [Glycine max (L.) Merrill] seed size. We investigated the influence of light enrichment and shading on seed growth rate, effective filling, cotyledon cell number, cell volume and endogenous ABA concentrations of cotyledons/testas during seed filling of soybean. Evans, an indeterminate Group 0 soybean, was subjected to light reduction and enrichment treatments from the beginning of pod formation until final harvest for two years in Massachusetts. Higher rates of seed growth, greater seed dry weight, and higher cotyledon cell number were all observed with light enrichment. There was a reduction in seed growth rate and cotyledon cell number, along with a significant lowering of endogenous ABA levels in testa and cotyledon with shade. The level of ABA in cotyledon during seed development was significantly correlated with seed growth rates only under shade treatments. Both the growth rates and seed filling duration were influenced by variation in light interception by the soybean canopy. The effects of varying light treatment on seed size, within one genotype, were most likely due to the differences in seed growth rate and cotyledon cell number.

Limited information is available regarding the source-sink alterations on soybean yield under whole plant light enriched conditions. The differential responses of yield components for two old and two new cultivars were investigated. The yield sensitivity of the old cultivars to the changes in source strength and light enriched conditions was much greater than that of the two new cultivars. The yield of the new cultivars was more likely sink-limited, and source-limited for the old cultivars. The increased yield by light enrichment was in part due to an increased branch contribution. Pod number per plant was more responsive than seed number per pod. The reduction in pod number by source-sink manipulation was more severe in the old cultivars. Light enrichment increased pod number in the two old cultivars but not in the two new cultivars. Seed size was responsive to changes in the source-sink ratio and changes in the environment. Both pod number or seed size may be increased or decreased if environmental conditions for seed filling are altered.

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Optimum plant population densities are a key means of achieving higher seed yield in soya bean [Glycine max (L.) Merr.]. Limited information is available on yield-density relation of glyphosate-resistant soya beans in north-eastern USA. The objective of this research was to determine the appropriate populations for glyphosate-resistant soya beans, and if the yield potential of glyphosate-resistant soya bean produced in light-enriched conditions was affected by populations. Eight glyphosate-resistant soya bean cultivars with three populations (300 000, 500 000 and 800 000 plants ha )1 ) were grown under both ambient and light-enriched conditions in 2002 and 2003. Yield of all cultivars responded to density linearly. As density increased, grain yield was increased by up to 92 % among cultivars. Light enrichment increased yield for all cultivars across the 2 years, although some cultivars were more sensitive. Harvest index either remained unchanged or declined slightly at higher density in 2002, and there was no difference among treatments in 2003. Both pod number and seed number per plant were significantly decreased with the increase of density across the 2 years, while seeds per pod declined slightly or remained unchanged. Greater seed size was obtained in higher density with varied degree depending on cultivars across the 2 years except for those cultivars with relatively larger seed. The increase in seed size by light enrichment was cultivar and density dependent, and varied between years. 800 000 plants ha )1 could be a suitable practice in producing higher yield in north-east USA for glyphosate-resistant soya bean. Maintaining the mass of an individual seed is an important strategy in achieving high yield at high population. Establishing mechanisms responsible for the greatest yields via high population under lightenriched conditions, may provide insights for management and phenotypic improvement.