Densely populated, intensively cropped highland areas in the tropics and subtropics are prone to erosion and declining soil fertility, making agriculture unsustainable. Conservation agriculture in its version of permanent raised bed planting with crop residue retention has been proposed as an alternative wheat production system for this agro-ecological zone. A five years field experiment comparing permanent and tilled raised beds with different residue management under rainfed conditions was started at El Batá n (Mexico) (2,240 m asl; 19.31°N, 98.50°W; Cumulic Phaeozem) in 1999. The objective of this study was to determine the soil quality status after five years of different management practices. The K concentration was 1.65 times and 1.43 times larger in the 0-5 cm and 5-20 cm profiles, respectively, for permanent raised beds compared to conventionally tilled raised beds. The Na concentration showed the opposite trend. Sodicity was highest for conventionally tilled raised beds and for permanent raised beds it increased with decreasing amounts of residue retained on the surface. Permanent raised beds with full residue retention increased soil organic matter content 1.4 times in the 0-5 cm layer compared to conventionally tilled raised beds with straw incorporated and it increased significantly with increasing amounts of residue retained on the soil surface for permanent raised beds. Soil from permanent raised beds with full residue retention had significantly higher mean weight diameter for wet and dry sieving compared to conventionally tilled raised beds. Permanent raised beds with full residue retention had significantly higher aggregate stability compared to those with residue removal. A lower aggregation resulted in a reduction of infiltration. Principal component analysis (PCA) was performed using these soil physicochemical variables that were significantly influenced by tillage or residue management. The PC1 and PC2 separated the conventionally tilled raised beds from the permanent raised beds and PC3 separated permanent raised beds with at least partial residue retention from permanent raised beds with no residue retention. These clear separations suggest that tillage and residue management have an effect on

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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 13 C ‰ signatures of the macroaggregates, microaggregates, the silt and Plant Soil (2008) 305:237-252

It is essential to have an indication of the sustainability of an agricultural system in addition to the potential, immediate benefits well before the catastrophic consequences of non-sustainability become apparent. Long-term experiments are best suited to test sustainability of a given system. This paper has a dual objective: (1) evaluate the Greenseeker handheld NDVI sensor as a tool for measuring within plot spatial variability, (2) address the question whether different management practices affect spatial within-plot crop growth variability and what this spatial variability tells us about the cropping system performance. Therefore, spatial and time variability of crop performance were measured during the 2004 and 2005 crop cycle for all plots of the different management treatments of a long-term (started 1991) tillage and residue management trial. The NDVI readings measured with the handheld sensor correlated well with the visual scoring in the field. The handheld sensor is time-efficient and gave reproducible results. The potential for using this tool to detect spatial crop variability, both within and between plots/ treatments, is promising. The coefficient of variation (CV) for the NDVI measurement sequence in each plot was determined. The CV's throughout the crop season reflected the canopy expansion and senescence curve of maize and wheat. The CV was high at the beginning of the crop season, however, once the canopy began to close, leaves from larger plants covered the leaves and whorl of smaller plants, extending further into the linear row. Measurements to investigate spatial variability related to crop performance should thus be done after this initial stage at the end of the vegetative period when the vegetative biomass of the crops is fully established. Zero tillage systems without surface residue retention produced high CVs of the NDVI sequence and high spatial crop variability throughout the season, even after the vegetative period. As the only factors differing between the different plots are the tillage/ Plant Soil (residue/rotation practices and as similar patterns were found for all plots representing repetitions of the same management practice (zero tillage without residue retention), increased variability is an indicator of agronomic mismanagement or, conversely, of sound agriculturally production practices.