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Aims: A field experiment was conducted where maintenance of indigenous arbuscular mycorrhizal (AM) fungal populations was attempted using AM host cover crops arranged temporally or spatially during growth of nonmycorrhizal crops. Methods: To arrange AM hosts temporally, sunflower or oat was grown as a cover crop after non-host cropping (cabbage) or fallowing. In order to arrange AM hosts spatially, red clover, white clover or vetch was intercropped during growth of non-host cabbage. Results: The AM colonization and growth of maize with previously introduced sunflower or oat were much greater than those without introduction of cover crops or those with introduction of non-host cover crops. The AM colonization and yield of winter wheat grown after cabbage with AM host intercropping were greater than those after cabbage only cropping, suggesting that arrangement of AM hosts between cabbage rows is effective for maintaining the AM fungal population in soil during non-host cropping. Conclusions: Mycorrhizal hosts cropped after or during non-host cropping is an effective means to increase indigenous AM fungal populations. The results show that AM colonization, P uptake and productivity of crops after cultivation of nonmycorrhizal crops can be improved by arranging AM hosts temporally or spatially as cover crops. [ABSTRACT FROM AUTHOR]

Croplands mainly act as net sources of the greenhouse gases carbon dioxide (CO) and nitrous oxide (NO), as well as nitrogen oxide (NO), a precursor of troposheric ozone. We determined the carbon (C) and nitrogen (N) balance of a four-year crop rotation, including maize, wheat, barley and mustard, to provide a base for exploring mitigation options of net emissions. The crop rotation had a positive net ecosystem production (NEP) of 4.4 ± 0.7 Mg C ha y but represented a net source of carbon with a net biome production (NBP) of -1.3 ± 1.1 Mg C ha y. The nitrogen balance of the rotation was correlated with the carbon balance and resulted in net loss (−24 ± 28 kg N ha y). The main nitrogen losses were nitrate leaching (−11.7 ±1.0 kg N ha y) and ammonia volatilization (−9 kg N ha y). Dry and wet depositions were 6.7 ± 3.0 and 5.9 ±0.1 kg N ha y, respectively. Fluxes of nitrous (NO) and nitric (NO) oxides did not contribute significantly to the N budget (NO: -1.8 ± 0.04; NO: -0.7 ± 0.04 kg N ha y) but NO fluxes equaled 16% of the total greenhouse gas balance. The link between the carbon and nitrogen balances are discussed. Longer term experiments would be necessary to capture the trends in the carbon and nitrogen budgets within the variability of agricultural ecosystems. [ABSTRACT FROM AUTHOR]

Permanent raised bed planting with crop residue retention is a form of conservation agriculture that has been proposed as an alternative to conventional tillage for wheat production systems in the Central Highlands of Mexico. A field experiment comparing permanent and tilled raised beds with different residue management under rainfed conditions was started at El Batán (State of Mexico, Mexico) in 1999. The percentage of small and large macroaggregates and mean weight diameter (MWD) was significantly larger in permanent raised beds compared to conventionally tilled raised beds both with full crop residue retention (average for maize and wheat), while the percentages free microaggregates was lower. The percentages of small and large macroaggregates and mean weight diameter (MWD) was significantly larger in permanent raised beds with residue retention compared to permanent raised beds with removal of the residue (average for maize and wheat), while the percentages free microaggregates and silt and clay fraction was lower. Cultivation of maize significantly reduced the large macroaggregates, while wheat reduced the silt and clay fraction (average over all systems). Cultivation of maize reduced the C and N content of the free microaggregates compared to soil cultivated with wheat, while removal of plant residue reduced the C and N content of the silt and clay fraction compared to soil where residue was retained. The C and N content of the coarse particulate organic matter (cPOM) and microaggregates within the macroaggregates was significantly larger in permanent raised beds compared to conventionally tilled raised beds both with full residue retention, while C and N content of the cPOM was significantly lower when residue was removed or partially removed compared to the soil where the residue was retained. The δ 13C ‰ signatures of the macroaggregates, microaggregates, the silt and clay fraction, cPOM and microaggregates within the macroaggregates were not affected by tillage or residue management when wheat was the last crop, but removal of residue reduced the δ 13C ‰ signatures of the macro-, microaggregates and microaggregates within the macroaggregates significantly compared to soil where the residue was retained. Retaining only 30–50% of the organic residue still improved the soil structure considerably compared to plots where it was removed completely. Permanent raised beds without residue retention, however, is a practice leading to soil degradation. [ABSTRACT FROM AUTHOR]

Using an alkaline calcareous soil, pot experiments were conducted to elucidate the effects of NHvs. NOnutrition (50 or 100 mg kg−1 soil) of wheat and maize on microbial activity in the rhizosphere and bulk soils. Dicyandiamide was used as nitrification inhibitor to maintain NHas the predominant N source for plants grown in NH-treated soil. While maize grew equally well on both N sources, root and shoot growth of wheat was higher under NHthan under NOnutrition. Bacterial population density on roots, but not in the rhizosphere soil, was higher under NHthan under NO supplied at 150 mg N kg−1 soil; whereas at both N levels applied, NHcompared to NOnutrition of wheat and maize significantly increased microbial biomass in the rhizosphere soil. Under both plant species, NH vs. NOnutrition also increased aerobic and anaerobic respiration, and dehydrogenase activity in the rhizosphere. As microbial activity in the planted bulk and unplanted soils was hardly affected by the N-source, we hypothesize that the stimulation by NHof the rhizosphere microbial activity was probably due to higher availability of root exudates under NH than under NOnutrition. [ABSTRACT FROM AUTHOR]