Cropping System and Nitrogen Effects on Mollisol Organic Carbon
Time, fertilizer, tillage, and cropping systems may alter soil organic carbon (SOC) levels. Our objective was to determine the effect of long-term cropping systems and fertility treatments on SOC. Five rotations and two N fertility levels at three Iowa sites (Kanawha, Nashua, and Sutherland) maintained for 12 to 36 yr were evaluated. A 75-yr continuous corn (Zea mays L.) site (Ames) with a 40-yr N-P-K rate study also was evaluated. Soils were Typic and Aquic Hapludolls and Typic Haplaquolls. Four-year rotations consisting of corn, oat (Avena sativa L.), and meadow (alfalfa [Medicago sativa L.], or alfalfa and red clover [Trifolium pratense L.]) had the highest SOC (Kanawha, 32.1 g/kg; Nashua, 21.9 g/kg; Sutherland, 27.9 g/kg). Corn silage treatments (Nashua, ≤ 18.9 g/kg; Sutherland, ≤23.2 g/kg) and no-fertilizer treatments (Kanawha, 25.3 g/kg; Nashua, ≤20.9 g/kg; Sutherland, ≤23.5 g/kg) had the lowest SOC. A corn-oat-meadow-meadow rotation maintained initial SOC (27.9 g/kg) after 34 yr at Sutherland. Continuous corn resulted in loss of 30% of SOC during 35 yr of manure and lime treatments. SOC increased 22% when N-P-K treatments were imposed. Fertilizer N, initial SOC levels, and previous management affected current SOC levels. Residue additions were linearly related to SOC (Ames, r2 = 0.40; Nashua, r2 = 0.82; Sutherland, r2 = 0.89). All systems had 22 to 49% less SOC than adjacent fence rows. Changing cropping systems to those that conserve SOC could sequester as much as 30% of C released since cropping began, thereby increasing SOC. 
Resource use at the cropping system level
This paper illustrates the basic ideas of good crop rotations, adequate crop husbandry and high resource-use efficiencies and some relevant ecological approaches. The use of special crops to prevent the need of high inputs of crop protectants or to reduce losses of nutrients at the level of the cropping system deserves special attention in research. Examples are given for the ecological control of soil-borne fungi, parasitic weeds, nitrogen loss and other sustainable techniques to increase the resource-use efficiency at the cropping system level. © 1997 Elsevier Science B.V. 
Nitrogen Efficiency Component Analysis: An Evaluation of Cropping System Differences in Productivity
The development of cropping systems that use N efficiently requires methods that evaluate system differences in N use. A procedure, based conceptually on soil and plant processes that affect N use, was developed to evaluate differences in N use efficiency among cropping systems. The method uses measurements of yield, grain N, aboveground plant N, applied N, and postharvest inorganic soil N to partition cropping system differences in yield and grain N into N efficiency components. The components consist of N supply, available N efficiency, available N uptake efficiency, N utilization efficiency, grain Naccumulation efficiency, and N harvest index. The N efficiency component analysis was demonstrated for a field study with hard red spring wheat (Triticum aestivum L. ‘WB 906R’) where conventional tillage had a greater yield and grain N than no-tillage. At low N rates, 78% of the difference in yield between the two was attributed to N supply and available N efficiency components. At high levels of applied N, 88% of the yield difference was attributed to the N utilization efficiency component. Differences in grain N were attributed to N supply and available N efficiency components, whereas components of grain N accumulation efficiency, available N uptake efficiency, and N harvest index were nonsignificant. Overall, this new approach transcends empirical analyses and provides insight into underlying mechanisms of cropping system differences in N use. 
Initial and Residual Effects of Organic and Inorganic Amendments on Soil Properties in a Potato-Based Cropping System in the Bolivian Andean Highlands
The objective of this study was to determine the effects of applications of organic and inorganic soil amendments on initial and residual soil chemical, physical and biological properties that may affect both short- and long-term soil fertility in a potato (Solanum tuberosum L.)-based cropping system of indigenous rural communities in the Bolivian Andean Highlands (Altiplano). Field experiments were conducted in four representative low and high elevation communities in the semi-arid Central Andean Region of Bolivia from 2006 to 2009. Treatments included a control, and applications of sheep and cow manure, a commercial household/urban compost product, a commercial biofertilizer soil amendment, urea and diammonium phosphate and combinations of these different treatments. Soil samples were taken from all the sites prior to application of treatments and planting of potatoes as well as during the growing season and prior to planting of a subsequent crop of quinoa (Chenopodium quinoa, Willd). Soil pH, soil total organic C, and total N increased due to application of organic fertilizers with or without inorganic fertilizers. Soil inorganic N and Bray-1 P were increased by inorganic fertilizers alone or when combined with organic fertilizers. The residual effect of most of the analyzed soil nutrients was detected in the subsequent growing season. In addition, lower soil bulk density was observed after organic fertilizers were applied with or without inorganic fertilizers and this residual effect persisted for the quinoa crop. In a controlled laboratory incubation experiment, soil potentially mineralizable C and N increased as organic fertilizers application rates rose from 0 to 30 Mg ha-1. These results highlight the importance of a balanced soil fertilization program in this region with use of optimum rates of both inorganic and organic soil amendments to increase short- and long-term soil fertility.. 
Effects of Time of Weed Removal and Cropping system on Weed Control and Crop Performance in Okra/Amaranthus Intercrop
A field trial was conducted during the late wet seasons of 2011 and 2012 at the Research Farm of the Federal University of Agriculture, Alabata, Abeokuta (7015’N, 3025’E) in the forest savanna- transition zone of Ogun State, South Western Nigeria. The objective was to evaluate the effect of time of weed removal and cropping system on weed control and crop performance in okra/amaranthus intercrop. The experiment was laid out in a Randomized Complete Block Design (RCBD) in a Split- plot arrangement. The treatments consisted of three main plots and five sub plots replicated three times. The main plot treatments were single hoe- weeding at 3 weeks after planting (WAP), double weeding at 3 & 6 WAP and no weeding, while the sub plots consisted of okra intercropped with amaranthus at 0.5g/m2or 1.0g/m2, okra sole and amaranthus sole at 0.5g/m2 or 1.0g/m2. Results from the study showed that intercropping of okra with amaranthus reduced weed infestation significantly (p<0.05) compared to sole okra. Weed control treatments significantly (p<0.05) reduced weed infestation in the intercrop while cropping system did not have any significant effect (p<0.05) on the weed biomass, plant height, pod length, number of pods and pod fresh weight. Uncontrolled weed infestation led to 50.7% yield loss in okra. It is therefore concluded that intercropping of okra with amaranthus is an effective means of reducing weed pressure in okra production as well as increasing land productivity. 
 Robinson, C.A., Cruse, R.M. and Ghaffarzadeh, M., 1996. Cropping system and nitrogen effects on Mollisol organic carbon. Soil Science Society of America Journal, 60(1), pp.264-269.
 Struik, P.C. and Bonciarelli, F., 1997. Resource use at the cropping system level. In Developments in Crop Science (Vol. 25, pp. 179-189). Elsevier.
 Huggins, D.R. and Pan, W.L., 1993. Nitrogen efficiency component analysis: an evaluation of cropping system differences in productivity. Agronomy Journal, 85(4), pp.898-905.
 Aguilera, J., Motavalli, P.P., Gonzales, M.A. and Valdivia, C., 2012. Initial and residual effects of organic and inorganic amendments on soil properties in a potato-based cropping system in the Bolivian Andean Highlands. Journal of Experimental Agriculture International, pp.641-666.
 Adeyemi, O.R., Fabunmi, T.O., Adedeji, V.O. and Adigun, J.A., 2014. Effects of time of weed removal and cropping system on weed control and crop performance in okra/Amaranthus intercrop. Journal of Experimental Agriculture International, pp.1697-1707.