Dryland Agroecosystems Research Articles

The potential for sand dams to increase the adaptive capacity of East African drylands to climate change

Drylands are home to more than two billion people and are characterised by frequent, severe droughts. Such extreme events are expected to be exacerbated in the near future by climate change. A potentially simple and cost-effective mitigation measure against drought periods is sand dams. This little-known technology aims to promote subsoil rainwater storage to support dryland agro-ecosystems. To date, there is little long-term empirical analysis that tests the effectiveness of this approach during droughts. This study addresses this shortcoming by utilising multi-year satellite imagery to monitor the effect of droughts at sand dam locations. A time series of satellite images was analysed to compare vegetation at sand dam sites and control sites over selected periods of drought, using the normalised difference vegetation index. The results show that vegetation biomass was consistently and significantly higher at sand dam sites during periods of extended droughts. It is also shown that vegetation at sand dam sites recovers more quickly from drought. The observed findings corroborate modelling-based research which identified related impacts on ground water, land cover, and socio-economic indicators. Using past periods of drought as an analogue to future climate change conditions, this study indicates that sand dams have potential to increase adaptive capacity and resilience to climate change in drylands. It therefore can be concluded that sand dams enhance the resilience of marginal environments and increase the adaptive capacity of drylands. Sand dams can therefore be a promising adaptation response to the impacts of future climate change on drylands.

Microbial Biomass Dynamics in a Tropical Agroecosystem: Influence of Herbicide and Soil Amendments

The influences of herbicide alone and in combination with the soil amendments with contrasting resource qualities on dynamics of soil microbial biomass C (MBC), N (MBN), and P (MBP) were studied through two annual cycles in rice-wheat-summer fallow crop sequence in a tropical dryland agroecosystem. The experiment included application of herbicide (butachlor) alone or in combination with various soil amendments having equivalent amount of N in the forms of chemical fertilizer, wheat straw, Sesbania aculeata, and farm yard manure (FYM). Soil microbial biomass showed distinct temporal variations in both crop cycles, decreased from vegetative to grain-forming stage, and then increased to maximum at crop maturity stage. Soil MBC was the highest in herbicide + Sesbania aculeata treatment followed by herbicide + FYM, herbicide + wheat straw, herbicide + chemical fertilizer, and herbicide alone treatments in decreasing order during the rice-growing period. During wheat-growing period and summer fallow, soil MBC attained maximum for herbicide + wheat straw treatment whereas herbicide + FYM, herbicide + Sesbania, and herbicide + chemical fertilizer treatments showed similar levels. The overall trend of soil MBN was similar to those of soil MBC and MBP except that soil MBN was higher in herbicide + chemical fertilizer treatment over the herbicide + wheat straw treatment during rice-growing period. In spite of the addition of equivalent amount of N through exogenous soil amendments in combination with the herbicide, soil microbial biomass responded differentially to the treatments. The resource quality of the amendments had more pronounced impact on the dynamics of soil microbial biomass, which may have implications for long-term sustainability of rainfed agroecosystems in dry tropics.

Soil microbial biomass dynamics in grassland and agroecosystem receiving varying resource quality soil inputs in dry tropics

Atwo year field experiments were conducted to evaluate the effect of land use conversion and management strategies variation through addition of varying resource quality soil inputs on soil microbial biomass dynamics, under rice-wheat-summer fallow crop sequence in tropical dryland agroecosystem. The treatments involved addition of equivalent amount of N through chemical fertilizer (CF) and three organic inputs, viz. Sesbania shoot (high quality resource, HQR), wheat straw (low quality resource, LQR) and Sesbania+wheat straw (mixed quality, MQR) besides control (CO) and grassland (GF). On annual mean basis, cultivation of grassland decreased the MBC (−51%) and MBN (−52%) level in soil. Relative to control, application of WS+SS considerably increased the level of MBC (+77%) and MBN (+87%) in soil, instead of SS, WS and CF. In all cultivated plots, a distinct seasonal and temporal variation in microbial biomass C and N were found contrary to GF. Both soil MBC and MBN level increased from rice to wheat period and reached maximum during summer fallow; during rice, the pattern in decreasing order was HQR>MQR>LQR>CF>CO; wheat period, MQR>LQR>HQR>CF>CO; summer fallow, MQR>LQR>HQR>CF>CO. It is concluded that cultivation exerts negative effects on the concentration of soil microbial biomass. Application of varying resource quality exogenous soil inputs maintains the microbial biomass level in soil, differentially. Addition of MOR was most effective for sustained buildup of microbial biomass in soil throughout the cycle, rather than HQR, LQR or CF. The fertility of soil in term of soil microbial biomass can be maintained by regulating the resource quality of exogenous inputs, and these results will help in designing the management strategies for sustaining long term soil fertility in these tropical dryland agroecosystem.

Earthworms: 'Soil and Ecosystem Engineers' – a Review

Earthworms can alter the soil environment by changing soil properties. They have great potentiality to enhance soil physical properties like bulk density, infiltrability, hydraulic conductivity, porosity, aggregate stability. Due to this ability they are only species which plays significant role in pedoturbation. Similarly, their role in nutrient cycling and organic matter breakdown is of unique interest. Earthworm cast fortified with the microbial population. Increasing microbial activity in soil, increases the nutrient mineralization and release. Earthworm activity enhances root distribution so that immobile macro nutrients like phosphorous and other micronutrients, which are absorbed by plant through root interception, are easily available to the plants. Hence, earthworms play important role in bioturbation, they are considered as 'soil engineer.' However, their activity differs with the agroecosystems. Their population density is more in reduced tillage system than conventional tillage system, aerobic condition than anaerobic conditions, grasslands than forest. Low population of earthworm was found in dry land agroecosystems. But earthworm presence in extreme environment is not studied well yet. So, this paper was reviewed to explore the role of earthworm as soil and ecosystem engineer. This was already established fact but main aim of this paper is to collect the related information and conclude the future research prospects to strengthen the earthworm role as soil and ecosystem engineer.

Grazing and no-till cropping impacts on nitrogen retention in dryland agroecosystems.

As the world's population increases, marginal lands such as drylands are likely to become more important for food production. One proven strategy for improving crop production in drylands involves shifting from conventional tillage to no-till to increase water use efficiency, especially when this shift is coupled with more intensive crop rotations. Practices such as no-till that reduce soil disturbance and increase crop residues may promote C and N storage in soil organic matter, thus promoting N retention and reducing N losses. By sampling soils 15 yr after a N tracer addition, this study compared long-term soil N retention across several agricultural management strategies in current and converted shortgrass steppe ecosystems: grazed and ungrazed native grassland, occasionally mowed planted perennial grassland, and three cropping intensities of no-till dryland cropping. We also examined effects of the environmental variables site location and topography on N retention. Overall, the long-term soil N retention of >18% in these managed semiarid ecosystems was high compared with published values for other cropped or grassland ecosystems. Cropping practices strongly influenced long-term N retention, with planted perennial grass systems retaining >90% of N in soil compared with 30% for croplands. Grazing management, topography, and site location had smaller effects on long-term N retention. Estimated 15-yr N losses were low for intact and cropped systems. This work suggests that semiarid perennial grass ecosystems are highly N retentive and that increased intensity of semiarid land management can increase the amount of protein harvested without increasing N losses.

Ecosystem services of native trees: experiences from two traditional agroforestry systems in Karnataka, Southern India

Agroforestry systems with native trees provide an interesting instance wherein provisional, regulatory, habitat, and ancillary ecosystem services are simultaneously exploited, but are least recognized. In the present study, the potential of native trees to optimize different ecosystem services is compared, versus introduced species in two traditional agroforestry systems in Karnataka state, Southern India: (a) coffee-based shaded perennial systems in Kodagu district of the tropical humid zone; and (b) dryland agro-ecosystems with Ficus trees in Mandya district of the semi-arid zone. Ecosystem services were assessed based on collected field data and farmers’ perceptions. Compared with exotic species, native trees provide more direct and indirect benefits, irrespective of differences in type, location, scope, and management of these systems. The role of native trees in supporting the overall sustainability of agroforestry systems is highlighted in the study. Moreover, it is emphasized that the long-term sustenance of traditional agroforestry systems depends on appreciation of the indirect and intangible benefits that are provided.

Diversity in weed seed production and the soil seed bank: Contrasting responses between two agroecosystems

This study was carried out to compare the diversity in seed production and the soil seed bank in a dryland and an irrigated agroecosystem in the dry tropics. Both agroecosystems showed a comparable number of species, but only 25% and 38% similarity during the winter and rainy cropping seasons, respectively. In the irrigated agroecosystem, the amount of seed production diversity was almost double in the winter season, compared to the rainy season. The weed seedbank diversity was low but was sensitive to cropping practices and seasons in both agroecosystems. A considerably smaller soil seedbank size in the irrigated agroecosystem (cf. dryland) was related to lowered weed seed production. The dryland agroecosystem showed a greater accumulation of the seeds of broad‐leaved weeds, whereas the irrigated agroecosystem accumulated more seeds of the grasses or sedges. About three‐fourths of the seeds during the winter season were accounted for by Anagallis arvensis and Chenopodium album in the dryland agroecosystem and by C. album and Melilotus indica in the irrigated agroecosystem. However, during the rainy season, Ammannia baccifera, Echinochloa colona and Cyperus rotundus dominated in both agroecosystems. The changes in the weed seed bank and its diversity are mainly attributed to differences in water management, which tends to reduce species diversity, especially at a lower depth, but leads to the dominance of some potentially noxious weeds (e.g. Phalaris minor and M. indica). Approximately double the soil seedbank size and a greater diversity at a lower depth might indicate an adaptive mechanism in the storage of weed seeds in the dryland agroecosystem.

Winter cereal root growth and aboveground–belowground biomass ratios as affected by site and tillage system in dryland Mediterranean conditions

Understanding the interaction between crop roots and management and environmental factors can improve crop management and agricultural carbon sequestration. The objectives of this study were to determine the response of winter cereal root growth and aboveground–belowground biomass ratios to tillage and environmental factors in the Mediterranean region and to test an alternative approach to determine root surface area. Winter cereal root growth and biomass ratios were studied in three sites with different yield potential according to their water deficit (high yield potential, HYP; medium yield potential, MYP; low yield potential, LYP) in the Ebro Valley (NE Spain). At all sites, three tillage systems were compared (conventional tillage, minimum tillage, no-tillage (NT)). Root surface density (RSD), soil water content, yield components, and grain yield were quantified and shoot-to-root and grain-to-root ratios were calculated. RSD was measured with the use of image analysis software comparing its performance to a more common intersection method. Significant differences on RSD between sites with different yield potential were found being the greatest at the HYP site and the lowest at the LYP one. Shoot-to-root ratio was 2.7 and 4.6 times greater at the HYP site than at the MYP and LYP sites, respectively. Moreover, the grain-to-root ratio was significantly affected by site, with a ratio that increased with yield potential. Tillage had no significant effects on RSD at any of the sites studied; however, tillage did affect grain yield, with NT having the greatest yields. This study shows that in the Mediterranean dryland agroecosystems, winter cereals relative above- and belowground biomass growth is strongly affected by the yield potential of each area. NT in the Mediterranean areas does not limit cereal root growth and leads to greater grain yields. A highly significant linear relationship (P < 0.001; r 2 0.77) was observed between the root surface values obtained with the free-software image analysis method and the most common intersection method, showing it to be a reliable method for quantifying root density.

Studi Keragaan Agroekosistem untuk Pengembangan Potensi Pertanian di Kabupaten Sukoharjo Propinsi Jawa Tengah

Title : The Study of Habitual Agroecosystem to Develope Agriculture Potential at Sukoharjo Regency in Central Java. The research about habitual agroecosystem to develope agriculture potential at Sukoharjo Regency in Central Java. This research conducted from June until September, 2002. The results showed that Sukoharjo Regency regional base on the Schmidt-Ferguson climate have C-climate type (slightly wet) and D (moderate), can be separate into four (4) agroecosystem, e.i wet land, dry land, mixedfarm, and forest. Wet land agroecosystem grouped into types agroecosystem with parent rock Merapi Volcano, Aluvium, Lawu ejecta, and Wonosari-Punung formation. Dry land agroecosystem grouped into types agroecosystem with parent rock Lawu ejecta, Aluvium, and Mandalika-Wonosari-Punung formation. Mixedfarm agroecosystem grouped into types agroecosystem with parent rock Lawu ejecta, and Mandalika-Wonosari-Punung formation. Whereas forest agroecosystem exist on the Wonosari-Punung formation, and it management strategy toward as community based conservation regional. Keywords : Agroecosystem, Agriculture potential, Sukoharjo Regency

Taxonomy and Distribution of Phenazine-Producing Pseudomonas spp. in the Dryland Agroecosystem of the Inland Pacific Northwest, United States

We investigated the taxonomic placement of phenazine-producing fluorescent Pseudomonas spp. in the Inland Pacific Northwest region of the United States. Five distinct species were identified, two of which were provisionally considered to be new. Agroclimatic zone and soil silt content affected the species diversity across the region.

Constraints of No‐Till Dryland Agroecosystems as Bioenergy Production Systems

Soil erosion and loss of soil organic C (SOC) may limit the sustainable harvest of crop residues for biofuels from dryland systems in the semiarid Great Plains. The objective of this study was to evaluate the capabilities and constraints of harvesting residues from dryland systems. The study used observations from a long‐term experiment in Colorado to examine biomass production from wheat (Triticum aestivum L.), corn (Zea mays L.), and grain sorghum [Sorghum bicolor (L.) Moench] at three no‐till sites in a winter wheat–corn–fallow (WCF) or winter wheat–sorghum–fallow cropping system. Modeling evaluated the impact of residue removal on erosion rates and SOC dynamics. The Revised Universal Soil Loss Equation and the Wind Erosion Equation were used to simulate erosion, and the DAYCENT model was used to estimate changes in SOC with residue removal. Biomass yield for WCF averaged 3.8 Mg ha−1, divided into stover and grain yields of 2.2 and 1.6 Mg ha−1. Water erosion was not shown to constrain residue harvest, but modeling indicated unsustainable wind erosion rates after removing 10 to 30% of corn residue. Simulations showed that up to 80% of wheat straw could be harvested without exceeding sustainable wind erosion rates. The major constraint to sustainable residue harvest is a residue return rate of 2.4 Mg ha−1 yr−1 of biomass to maintain SOC.

Yield-increase effects via improving soil phosphorus availability by applying K2SO4 fertilizer in calcareous–alkaline soils in a semi-arid agroecosystem

Many studies have reported evidence describing the effects of K2SO4 fertilizer on crop productivity, but there is scant information about the yield-increasing mechanisms when influencing soil properties by K2SO4 application in calcareous and alkaline soils. In this study, one field and incubation experiments were conducted to investigate the effects of K2SO4 fertilizer on crop yields and soil properties in calcareous and alkaline soils on the Loess Plateau of Northwestern China. In field experiments, four K2SO4 treatments were applied to potato (Solanum tuberosum L.) in 2007 and spring wheat (Triticum aestivum L.) in 2008: (1) CK: no K2SO4; (2) T1: K2SO4 @ 100kgha−1 in 2007 and 50kgha−1 in 2008; (3) T2: K2SO4 @ 200kgha−1 in 2007 and 100kgha−1 in 2008; and (4) T3: K2SO4 @ 300kgha−1 in 2007 and 150kgha−1 in 2008. In 2007, potato yield increased by 17.4% in T2 and 21.5% in T3 compared with CK, but did not significantly increase in T1. In 2008, spring wheat yields increased by 10.0%, 15.8% and 18.7% in T1, T2 and T3 treatments, respectively, compared with CK. Stepwise regression (P≤0.05) revealed that soil-available K at tuber formation and starch accumulation stage, and available P at starch accumulation stage correlated well with potato yield. Soil available P before sowing and at anthesis correlated well with spring wheat yield. Soil available P content was mostly higher in T1, T2 and T3 than in CK from June 2007 to August 2008 when the same dose P fertilizer was applied in all plots. Applying K2SO4 decreased soil pH. Soil available P was significantly negatively correlated with soil pH (R=–0.5721, P=0.0015). In an incubation experiment, the four K2SO4 treatments were designed: (1) CK: no K2SO4; (2) S1: K2SO4 @ 0.44gkg−1 dry soil; (3) S2: K2SO4 @ 0.88gkg−1 dry soil; (4) S3: K2SO4 @ 1.32gkg−1 dry soil. The results also showed that addition of K2SO4 significantly decreased soil pH and increased available P in calcareous and alkaline soils. Our study suggests that K2SO4 is desirable for improving crop productivity by increasing soil P availability via decreasing soil pH in calcareous and alkaline soils besides K effect in a low input dryland agroecosystem.

Social–ecological traps and transformations in dryland agro-ecosystems: Using water system innovations to change the trajectory of development

Recent efforts to achieve a much needed productivity increase in farming systems across semi-arid and dry sub-humid sub-Saharan Africa have highlighted the potential of small-scale water system innovations (SWSIs). This paper takes a social–ecological resilience approach to investigate how this type of water management technology would influence agro-ecosystem dynamics, using a catchment in northeastern Tanzania as an example. The analysis finds that three external drivers (increasing dryspell frequency, population growth, and institutional changes) have interacted with a set of key variables in the studied system to shape a development trajectory over the past half-century where off-farm ecosystem services are being degraded while agricultural yields remain low and people remain poor. The analysis further finds that the evaluated SWSIs have the potential to destabilize feedbacks maintaining this social–ecological trap through several different mechanisms, and thereby open up for new development trajectories. A concluding discussion identifies a number of challenges to this type of transformation in sub-Saharan Africa, and outlines the type of investment approaches that would be needed to go from potential to reality.

Influence of Herbicide and Soil Amendments on Soil Nitrogen Dynamics, Microbial Biomass, and Crop Yield in Tropical Dryland Agroecosystems

Nitrogen (N) dynamics is the key factor for designing management strategies to achieve sustained crop productivity. A 2‐yr field experiment was performed to examine the influence of herbicide (butachlor [2‐chloro 2′6′ diethyl‐n‐butoxy‐methylacetanilide]; HC) alone or in combination with soil amendments on N dynamics involving N mineralization rate (Nmin), inorganic N pool, N uptake (Nup) by plants, soil microbial biomass N, and crop yield in tropical dryland agroecosystems. The Nmin rate showed distinct seasonal patterns which decreased from rice period to summer fallow except herbicide + animal manure (farmyard manure; HC + AM) and herbicide + crop residue (wheat [Triticum aestivum L.] straw; HC + CR) treatments.The annual rate of Nmin, inorganic N pool, and crop yield were higher in HC + AM, herbicide + chemical fertilizer (NPK; HC + CF), herbicide + green manure {dhaincha [Sesbania aculeata (Willd.) Pers.] shoots; HC + GM} while comparable in HC + CR and HC treatments relative to control (CO). Soil moisture content better predicted variation in Nmin rate compared to soil temperature. Plant Nup and crop yield relative to CO was higher in HC + CF, HC + GM, and HC + AM, comparable in HC + CR while lower in the HC treatment. Annual Nmin rate explained a significant part of variations in annual soil microbial biomass N, inorganic N pool, plant Nup, and crop yield. Results indicate that combined application of HC + AM result in relatively sustained availability of inorganic N and higher crop yield throughout the annual cycle.

Application of fractal theory in assessing soil aggregates in Indian tropical ecosystems

Soil aggregation varies according to climatic and management factors, and is difficult to measure because of irregular shapes and sizes of soil aggregates. We applied fractal theory to assess soil aggregation as affected by (1) land use change from forest to savanna, (2) nutrient additions in forest, ecotone, and savanna ecosystems, and (3) tillage practice and residue treatments in an agro-ecosystem. We used fractal dimensions nonlinear (Dnon-lin) and linear (Dlin) based on number of aggregates (N) and mass of aggregates (M) (the range of values were 2.6–2.89 and 2.69–3.41, respectively) to capture the variations in the sizes of soil aggregates due to land use and treatments/management in these ecosystems. The variation in the values of non-linear fractal dimension based on mass (DMnon-lin) was smaller in forest and savanna ecosystems with and without nutrient additions, while the variation was wider in agro-ecosystems with different management practices. Linear fractal dimensions based on number (DNlin) and mass (DMlin) of aggregates varied marginally in these ecosystems and did not capture the variations in soil aggregates well. The variations in non-linear fractal dimension indicate that continued nitrogen loading in forest accelerates the formation of macro-aggregates, whereas in savanna the situation was reversed. The values of non-linear fractal dimensions did not show significant change after 6 years of nutrient additions in the ecotone; reflecting a buffering mechanism of this system in soil aggregate formation. On the basis of non-linear fractal dimension values, we conclude that residue retention and minimum tillage are appropriate for proper maintenance of soil aggregate stability for sustained crop production in the Indian dry land agro-ecosystems.

Soil organic carbon storage in a no-tillage chronosequence under Mediterranean conditions

The duration of soil organic carbon (SOC) sequestration in agricultural soils varies according to soil management, land-use history and soil and climate conditions. Despite several experiments have reported SOC sequestration with the adoption of no-tillage (NT) in Mediterranean dryland agroecosystems scarce information exists about the duration and magnitude of the sequestration process. For this reason, 20 years ago we established in northeast Spain a NT chronosequence experiment to evaluate SOC sequestration duration under Mediterranean dryland conditions. In July 2010 we sampled five chronosequence phases with different years under NT (i.e., 1, 4, 11, and 20 years) and a continuous conventional tillage (CT) field, in which management prevailed unchanged during decades. Soil samples were taken at four depths: 0–5, 5–10, 10–20 and 20–30 cm. The SOC stocks were calculated from the SOC concentration and soil bulk density. Furthermore, we applied the Century ecosystem model to the different stages of the chronosequence to better understand the factors controlling SOC sequestration with NT adoption. Differences in SOC stocks were only found in the upper 5 cm soil layer in which 4, 11 and 20 years under NT showed greater SOC stocks compared with 1 year under NT and the CT phase. Despite no significant differences were found in the total SOC stock (0–30 cm soil layer) there was a noteworthy difference of 5.7 Mg ha−1 between the phase with the longest NT duration and the phase under conventional tillage. The maximum annual SOC sequestration occurred after 5 years of NT adoption with almost 50% change in the annual rate of SOC sequestration. NT sequestered SOC over the 20 years following the change in management. However, more than 75% of the total SOC sequestered was gained during the first 11 years after NT adoption. The Century model predicted reasonably well SOC stocks over the NT chronosequence. In Mediterranean agroecosystems, despite the continuous use of NT has limited capacity for SOC sequestration, other environmental and agronomic benefits associated to this technique may justify the maintenance of NT over the long-term.

Biosolids application to no-till dryland agroecosystems

Dryland agroecosystems are generally ideal environments for recycling biosolids. However, what is the efficacy of biosolids addition to a no-till dryland management agroecosystem? From 2000 to 2010, we studied application of biosolids from the Littleton/Englewood, CO Wastewater Treatment Plant versus commercial N fertilizer in dryland no-till wheat (Triticum aestivum, L.)–fallow (WF) and wheat–corn (Zea mays, L.)–fallow (WCF) rotations at a site approximately 40km east of Byers, CO. We tested if biosolids would produce the same yields and grain P, Zn, and Ba concentrations as an equivalent rate of N fertilizer, that biosolids-borne P, Zn, and Ba would not migrate below the 10cm soil depth, and that biosolids application would result in the same quantity of residual NO3–N as the equivalent N fertilizer rate. Biosolids and N fertilizer produced similar wheat and corn yields; but, biosolids application resulted in smaller wheat grain Ba due to the soil formation of BaSO4. Biosolids application produced greater NO3–N concentrations than N fertilizer in the 30–60 and 60–90cm depths for the WF rotation and all but the 5–10 and 120–150cm depths for the WCF rotation. We concluded that biosolids application in a no-till managed dryland agroecosystem is an efficacious method of recycling this nutrient source.

Modelling tillage and nitrogen fertilization effects on soil organic carbon dynamics

Agricultural management plays an important role in global warming mitigation due to its effects on soil organic carbon (SOC) dynamics. In Mediterranean agroecosystems, the interactive effects of tillage and N fertilization on SOC storage have scarcely been studied. Hence, we here present a modelling study in which the effects of both tillage and N fertilization on SOC dynamics are investigated. We used SOC and C input data from a long-term (13 years) field study located in northeast Spain, firstly to validate both the Century model and the Rothamsted Carbon (RothC) model and secondly to predict future SOC dynamics until the year 2030. Tillage and N fertilization affected SOC stocks in the 0–30cm soil layer. However, the interaction of the two factors was not significant. Averaged over the three N fertilization rates, the observed mean SOC stocks in conventional tillage (CT) and no-tillage (NT) were 29.8 and 36.8MgCha−1, respectively. In addition, the observed SOC stocks, averaged for both tillage systems, increased with increasing N rates, with 30.6, 33.5 and 35.8MgCha−1 for the 0, 60 and 120kgNha−1 rates, respectively. In general, both the Century model and the RothC model performed well in predicting SOC dynamics. Model predictions showed that in Mediterranean dryland agroecosystems SOC dynamics in the next 20 years would be variable according to the tillage and N fertilization applied. According to these predictions, scenarios with NT and high fertilization rates (e.g., 60–120kgNha−1) could lead to significant SOC sequestration and associated CO2 emission offsetting. However, these scenarios with high SOC sequestration rates also showed high mineral N accumulation in the soil profile with its associated environmental side effects.

Symbiotic nitrogen fixation and soil N availability under legume crops in an arid environment

Purpose Legume crops often present an important option to maintain and improve soil nitrogen (N) quality and fertility in a dryland agroecosystem. However, the work on the integral assessment of the symbiotic N2 fixation (Nfix) and their effects on soil N availability under field conditions is scare.

High specific activity in low microbial biomass soils across a no-till evapotranspiration gradient in Colorado

Abstract The need to identify microbial community parameters that predict microbial activity is becoming more urgent, due to the desire to manage microbial communities for ecosystem services as well as the desire to incorporate microbial community parameters within ecosystem models. In dryland agroecosystems, microbial biomass C (MBC) can be increased by adopting alternative management strategies that increase crop residue retention, nutrient reserves, improve soil structure and result in greater water retention. Changes in MBC could subsequently affect microbial activities related to decomposition, C stabilization and sequestration. We hypothesized that MBC and potential microbial activities that broadly relate to decomposition (basal and substrate-induced respiration, N mineralization, and β-glucosidase and arylsulfatase enzyme activities) would be similarly affected by no-till, dryland winter wheat rotations distributed along a potential evapotranspiration (PET) gradient in eastern Colorado. Microbial biomass was smaller in March 2004 than in November 2003 (417 vs. 231 μg g−1 soil), and consistently smaller in soils from the high PET soil (191 μg g−1) than in the medium and low PET soils (379 and 398 μg g−1, respectively). Among treatments, MBC was largest under perennial grass (398 μg g−1). Potential microbial activities did not consistently follow the same trends as MBC, and the only activities significantly correlated with MBC were β-glucosidase (r = 0.61) and substrate-induced respiration (r = 0.27). In contrast to MBC, specific microbial activities (expressed on a per MBC basis) were greatest in the high PET soils. Specific but not total activities were correlated with microbial community structure, which was determined in a previous study. High specific activity in low biomass, high PET soils may be due to higher microbial maintenance requirements, as well as to the unique microbial community structure (lower bacterial-to-fungal fatty acid ratio and lower 17:0 cy-to-16:1ω7c stress ratio) associated with these soils. In conclusion, microbial biomass should not be utilized as the sole predictor of microbial activity when comparing soils with different community structures and levels of physiological stress, due to the influence of these factors on specific activity.