Rolf Sommer

Carbon storage and root penetration in deep soils under small-farmer land-use systems in the Eastern Amazon region, Brazil

The north-east of Pará state in the Eastern Amazon of Brazil was settled over 100 years ago. Today the region is an agricultural landscape with variously-aged secondary vegetation and fields with annual cultures, plantation crops and pastures. The effect of these different land covers on carbon sequestration as well as on water and nutrient extraction remain subject of debate. Therefore, we assessed the importance of land use on soil carbon stocks by measuring various C fractions and root biomass (0–6 m) in slash-and-burn systems and (semi-) permanent cultures. An extensive root system down to at least 6 m depth was present under various secondary vegetation stands and slashed and burned fields recently taken into cultivation as well as under a primary forest. Shallower rooting patterns were evident under (permanent) oil palm (4.5 m) and (semi-permanent) passion fruit plantations (2.5 m). Carbon storage in soils of traditional slash-and-burn agriculture up to 6 m depth (185 t ha-1) was not significantly lower than under a primary forest (196 t ha-1) but declined significantly under (semi-) permanent cultures (to 146–167 t ha-1). Compared to above-ground C losses, soil C losses due to slash-and-burn agriculture may thus be small. This is an argument for maintaining the secondary vegetation as part of the agricultural land-use system, as the root system of its trees is conserved and thus C is sequestered also at greater depth.

Modeling irrigated cotton with shallow groundwater in the Aral Sea Basin of Uzbekistan: II. Soil salinity dynamics

Years of ill-managed irrigation have triggered secondary soil salinization in the Khorezm region of Uzbekistan located in the Aral Sea basin. To assess the magnitude and dynamics of secondary soil salinization, to quantify improved management strategies and to derive updated irrigation standards, the soil water model Hydrus-1D was used. Water and soil salinity dynamics in three cotton fields with different soil textures were monitored and simulated for the years 2003 and 2005. Until now in Khorezm, overall soil salinity could only be controlled by pre-season salt leaching using high amounts of water. This water, however, may not be available anymore in the near future because of global climate change and shrinking fresh water resources. Simulations confirmed that the present leaching practice is barely effective. At two out of the three locations within a sandy loam field, leaching did not remove salts from the 2xa0m profile. Instead, salts were only shifted from the upper (0–0.8xa0m) to the lower (0.8–2xa0m) soil layer. Strong groundwater contribution to evapotranspiration triggered secondary (re)-salinization of the topsoil during the cropping season. As a consequence, salt amounts in the top 0.8xa0m of soil increased from 9 to 22xa0Mgxa0ha−1 in the field with loamy texture, and from 4 to 12xa0Mgxa0ha−1 in the field with sandy texture. Management strategy analyses revealed that reducing soil evaporation by a surface residue layer would notably decrease secondary soil salinization. Here, owing to the reduced capillary rise of groundwater, post-season salt contents of the three fields were reduced by between 12 and 19% when compared with residue-free conditions. Even more effective would be improving the efficiency of the drainage system so as to lower the groundwater table. This would require a revision of the current irrigation management schemes, but could, as simulations revealed, reduce the post-season salt content in the 2xa0m soil profile of the three fields by between 36 and 59% when compared with unaltered conditions. For the revised irrigation management in total not more water than already foreseen by national irrigation recommendations would be needed. Increasing leaching and irrigation efficiency would help sustaining the present cotton production levels while reducing future leaching demands.

Nutrient balance of shifting cultivation by burning or mulching in the Eastern Amazon – evidence for subsoil nutrient accumulation

For over a hundred years shifting cultivation with slash-and-burn land preparation has been the predominant type of land use by smallholders in the Bragantina region of the Brazilian Eastern Amazon. This study contrasts the nutrient balance of slash-and-burn agriculture with a fire-free cultivation. Therefore, one half of a 3.5-year-old (28.7 t DM ha–1) and a 7-year-old woody fallow vegetation (46.5 t DM ha–1) was burnt and the other half mulched, leaving the biomass as a surface residue. Subsequently, a sequence of maize, beans and cassava was cropped for 1.5 year. Burning the 3.5- and 7-year-old fallow removed 97 and 94% of the C, 98 and 96% of the N, 90 and 63% of the P-stocks, and between 45 and 70% of the cations K, Mg and Ca of the aboveground biomass by volatilization or ash-particle transfer. These losses were avoided with the slash-and-mulch land preparation. Mulching did not increase the losses of nutrients by leaching, despite the high amount of rapidly decomposing surface mulch. Also the length of preceding fallow had no significant influence on leaching losses. At a depth of 3 m, leached nutrients were quantitatively negligible in both treatments. Comparing the nutrient fluxes at soil depths of 0.9 m, 1.8 m and 3 m, the amounts of all mobile nutrients, and also of chloride and sodium were markedly reduced during percolation and must have been retained. It is likely that nutrient retention in the subsoil layer is only temporary, emphasizing the need for a rapid re-establishment of the naturally deep-rooting secondary vegetation after abandonment of sites to enable uptake of these nutrients. The overall nutrient balance was highly negative for slash-and-burn. 291 and 403 kg N ha–1, 21 and 18 kg P ha–1, and 70 and 132 kg K ha–1 were removed from the burnt plots with a preceding fallow of 3.5 and 7 years, respectively. A reduced fallow period (3.5 years), which is a common trend in the region, resulted in a higher mean annual rate of nutrient loss averaged over the duration of the cycle than a fallow period of 7 years. Eliminating the burning losses by mulching brought the agricultural system back to an equilibrated or even slightly positive nutrient balance, even after a reduced fallow period. Thus, slash-and-mulch is a viable alternative to maintain agricultural productivity and ecosystem functioning.

Transpiration and canopy conductance of secondary vegetation in the eastern Amazon

Abstract Secondary woody vegetation in the Brazilian Amazon accounts for about 30% of the cleared area in this region, which exceeds 100,000xa0km 2 . Despite the relative predominance of secondary vegetation, the hydrological and climatic properties of these areas are not well documented. In an effort to address this, the evapotranspiration ( E ) of a 3.5-year-old secondary vegetation in the eastern Amazon of Brazil was measured over 1 year. The annual evapotranspiration according to the Bowen ratio energy balance (BREB) amounted to 1421xa0mm, which is equal to rates quoted for tropical primary forests. The secondary vegetation returned 73% of the annual rainfall (1954xa0mm) to the atmosphere. Evapotranspiration required 73% of the net-radiation ( R n ) energy; this function remained fairly constant over the whole year. In order to estimate evapotranspiration with the Penman–Monteith (PM) method, the canopy conductance ( g c ) was determined using the BREB results. The monthly mean daily g c varied between 14 and 22xa0mmxa0s −1 in the rainy season and the transitional period (January–August), and reached a minimum of 7xa0mmxa0s −1 in the dry season in October. The hourly as well as daily mean canopy conductance were approximated by a multiple linear regression analysis incorporating hourly and daily averages of R n and vapour pressure deficit, respectively. In addition, the Jarvis-type model, which is based on a set of environmental control functions, was applied to predict hourly g c . The multiple linear regression and the non-linear optimisation (Jarvis-type model) were equally suitable for g c prediction. The optimised environmental control functions were comparable to those predicted elsewhere for Amazonian primary forests. This underlines the similarity of secondary and primary forests with respect to hydrological characteristics as well as energy turnover.

Modeling irrigated cotton with shallow groundwater in the Aral Sea Basin of Uzbekistan: I. Water dynamics

In Khorezm, a region located in the Aral Sea basin of Uzbekistan, water use for irrigation of predominantly cotton is high whereas water use efficiency is low. To quantify the seasonal water and salt balance, water application, crop growth, soil water, and groundwater dynamics were studied on a sandy, sandy loam and loamy cotton field in the years 2003 and 2005. To simulate and quantify improved management strategies and update irrigation standards, the soil water model Hydrus-1D was applied. Results showed that shallow groundwater contributed a substantial share (up to 399xa0mm) to actual evapotranspiration of cotton (estimated at 488–727xa0mm), which alleviated water stress in response to suboptimal quantities of water applied for irrigation, but enhanced concurrently secondary soil salinization. Thus, pre-season salt leaching becomes a necessity. Nevertheless, as long as farmers face high uncertainty in irrigation water supply, maintaining shallow groundwater tables can be considered as a safety-net against unreliable water delivery. Simulations showed that in 2003 around 200xa0mm would have been sufficient during pre-season leaching, whereas up to 300xa0mm of water was applied in reality amounting to an overuse of almost 33%. Using some of this water during the irrigation season would have alleviated season crop-water stress such as in June 2003. Management strategy analyses revealed that crop water uptake would only marginally benefit from a permanent crop residue layer, often recommended as part of conservation agriculture. Such a mulch layer, however, would substantially reduce soil evaporation, capillary rise of groundwater, and consequently secondary soil salinization. The simulations furthermore demonstrated that not relying on the contribution of shallow groundwater to satisfy crop water demand is possible by implementing timely and soil-specific irrigation scheduling. Water use would then not be higher than the current Uzbek irrigation standards. It is argued that if furrow irrigation is to be continued, pure sandy soils, which constitute <5% of the agricultural soils in Khorezm, are best to be taken out of annual cotton production.

Energy Use and CO 2 Production in Tropical Agriculture and Means and Strategies for Reduction or Mitigation

Carbon dioxide emissions due to fossil fuel consumption are well recognized as a major contributor to climate change. In the debate on dealing with this threat, expectations are high that agriculture based economies of the developing world can help alleviate this problem. But, the contribution of agricultural operations to these emissions is fairly small. It is the clearing of native ecosystems for agricultural use in the tropics that is the largest non-fossil fuel source of CO2 input to the atmosphere.Our calculation show that the use of fossil energy and the concomitant emission of CO2 in the agricultural operational sector - i.e. the use of farm machinery, irrigation, fertilization and chemical pesticides - amounts to merely 3.9% of the commercial energy use in that part of the world. Of this, 70% is associated with the production and use of chemical fertilizers. In the absence of fertilizer use, the developing world would have converted even more land for cultivation, most of which is completely unsuitable for cultivation. Current expectations are that reforestation in these countries can sequester large quantities of carbon in order to mitigate excessive emissions elsewhere. But, any program that aims to set aside land for the purpose of sequestering carbon must do so without threatening food security in the region. The sole option to liberate the necessary land for carbon sequestration would be the intensification of agricultural production on some of the better lands by increased fertilizer inputs.As our calculations show, the sequestration of carbon far outweighs the emissions that are associated with the production of the extra fertilizer needed. Increasing the fertilizer use in the developing world (without China) by 20%, we calculated an overall net benefit in the carbon budget of between 80 and 206 Mt yr−1 dependent on the carbon sequestration rate assumed for the regrowing forest. In those regions, where current fertilizer use is low, the relative benefits are the highest as responding yield increases are highest and thus more land can be set aside without harming food security. In Sub-Saharan Africa a 20% fertilizer increase, which amounts to 0.14 Mt of extra fertilizer, can tie up somewhere between 8 and 19 Mt of CO2 per year (average: 96 t CO2 per 1 t fertilizer). In the Near East and North Africa with a 20%-increased fertilizer use of 0.4 Mt yr-1 between 10 and 24 Mt of CO2 could be sequestered on the land set aside (40 t CO2 per 1 t fertilizer). In South Asia this is 22–61 Mt CO2 yr−1 with an annual additional input of 2.15 Mt fertilizer (19 t CO2 per 1 t fertilizer).In fact, carbon credits may be the only way for some of the farmers in these regions to afford the costly inputs. Additionally, in regions with already relatively high fertilizer inputs such as in South Asia, an efficient use of the extra fertilizer must be warranted.Nevertheless, the net CO2 benefit through implementation of this measure in the developing world is insignificant compared to the worldwide CO2 output by human activity. Thus, reforestation is only one mitigating measure and not the solution to unconstrained fossil fuel CO2 emissions. Carbon emissions should, therefore, first of all be reduced by the avoidance of deforestation in the developing world and moreover by higher energy efficiency and the use of alternative energy sources.

Chapter 2 Nitrogen in Rainfed and Irrigated Cropping Systems in the Mediterranean Region

Abstract The Middle East region, embracing West Asia and North Africa (WANA), is characterized by a Mediterranean climate which dictates the agriculture of the region. Being one of the centers of origin of settled farming, where cereals and pulses originated along with sheep and goats, crop production is largely rainfed and is dependent on the limited rainfall (200–600xa0mm) during the cool moist season in late fall to late spring. The agricultural sector has changed in recent decades with intensification and pressure on land use. As elsewhere, increased chemical fertilizer use, especially nitrogen (N), has had an impact on the transition from a traditional system to a more market driven one with increased inputs. This review examines the varied aspects of N in the soils and cropping systems as reflected by research at The International Center for Agricultural Research in the Dry Areas (ICARDA) in Syria in collaboration with other countries of the West Asia–North Africa region, especially in Morocco and other countries north and south of the Mediterranean. The synthesis, therefore, reflects a broad overview of conditions that impinge an N nutrition of crops and the evolution of N research achievements since the advent of commercial fertilization over three decades ago. With few exceptions, the soils of the Mediterranean region are low in organic matter and consequently in the reserves of total N, thus posing a limit of growing crops without fertilizer N or biological N fixation (BNF) through legumes. Soil calibration studies established the value of the soil nitrate test as a predictor of crop response with field trials to establish application rates for the main crops. Applicability is influenced by depth of sampling and the extent of mineralization. Dryland crop responses to N varied widely throughout the region from 30 to 150xa0kg Nxa0ha −xa01 , being dependent on soil N status and seasonal rainfall as the major determinant of yields. Splitting the N application was only advantageous in higher rainfall areas. Residual N from BNF by food and forage legumes influenced soil N supply for cereals and relative responses to N fertilizer. The contribution of rhizobia fixation to all the major legumes was quantified using 15 N along with management factors that influenced BNF by legumes. Where legumes were newly introduced to a region, rhizobial inoculation was considered necessary. With cereal responses to fertilizer N, differences between varieties were highlighted. Where urea or ammonium–N fertilizers were used, volatilization was the main loss mechanisms rather than leaching or denitrification. Considerable work was done on N use within crop rotation systems and components of the N cycle defined along with inputs from urine and feces from grazing animals. Forage legumes were shown to enhance total soil N and both labile and biomass N, with the least influence from fallow. These N forms were shown to fluctuate during the year as moisture and temperature conditions changed. Fertilizer N use had a positive effect on grain quality with increased protein, as well as soil organic matter (SOM) and thus soil quality. The significant change of the gradual introduction of supplemental irrigation in traditional rainfed cropping areas and its implications for use of models to describe the complex nature of N in dryland cropping systems was described. With the likelihood of a continuation of intensification of the dryland cropping systems in the Mediterranean region, N fertilizer use will inevitably increase and along with it the need for greater use efficiency in the interest of production economics and the environment. While limited use has been made of modeling of N, this approach is likely to be of more significance in integrating the varied facets of N under Mediterranean cropping conditions.

Organic carbon in soils of Central Asia—status quo and potentials for sequestration

Expectations have been raised that carbon sequestration in soils could provide a short-term bridge to reduce the impacts of increasing carbon emissions until low-carbon technologies are available. To assess the role of Central Asia in this regard, the organic carbon in soils of Central Asia and losses in response to land use were quantified in a spatially explicit way. Based on literature information on soil organic carbon contents and in combination with the FAO-UNESCO Soil Map of the Word, the organic carbon stocks in the upper 30xa0cm of native soils of Central Asia were estimated to amount to 20,17u2009±u20094,03xa0Pg. The extent of conversion of native land into agricultural land and the degradation of rangelands was assessed by a land use land cover change map of the region. This type of land use (change) was responsible for a reduction of the soil organic carbon by about 828u2009±u2009166xa0Tg C, or on average 4.1% of the total stocks. To this reduction, degradation of rangeland (observed on 4.9xa0Mha) with 50xa0Tg contributed only 6%. Most of the losses resulted from past conversion of rangelands into rainfed or irrigated agricultural land in the north of Kazakhstan. Hotspots of high soil organic matter depletion were former wetlands, drained for cultivation during the last decades. Assuming that improved agronomic and grazing management could be put in place and that therewith SOC levels in all of Central Asia’s cropland and degraded rangeland could be brought back to native levels in the next 50xa0years, each year 16.6xa0Tg C could be sequestered. This is equal to the sizeable amount of 15.5% of the 2004 annual anthropogenic C-emissions of the five Central Asian countries (107xa0Tg C yr−1). However, Central Asia contributed only 1.4% of CO2 that is set free worldwide by fossil fuel burning. Therefore, the mitigation effect on rising atmospheric CO2 levels and climate change of such ambitious sequestration plans, if put into practice, would be hardly notable. The central Asian example shows that, unfortunately, the strategy of soil C sequestration as a stand-alone measure is not a viable bridge to a future in which alternative energy source can substitute fossil fuel burning, but can only be part of a set of mitigating measures.

Chapter three – Significance of Phosphorus for Agriculture and the Environment in the West Asia and North Africa Region

Abstract Fertilizers have been largely responsible for the massive increases in world food production in the past half century that permitted accelerated global population growth to current unprecedented levels. Fertilizer use not only impacts crop yields but also affects animal production. While nitrogen (N) has been the main driver of such changes, phosphorus (P) also has a major role. Like N, the use of P fertilizers can have implications beyond the farmers’ fields, if excessive amounts are applied. The past four decades have witnessed overuse of P fertilizers as well as animal manures in the intensive agricultures of some European countries and North America. Yet ironically in many areas of the world, notably Africa, agricultural output is largely constrained by low soil P in combination with little or no P fertilizer application. Rock phosphate is the global source of the raw material for P fertilizer. However, resources are finite, and therefore efficient and wise use is of paramount importance. The vast West Asia and North Africa (WANA) region is one where agricultural output is beset with major environmental constraints. Yet fertilizer use in the region is still in the incipient to early development stage, ironically in view of the fact that major deposits of exploitable rock phosphate are found in the region, mainly in Morocco and Tunisia. With the predominantly calcareous soils of the region being inherently low in available P, the main focus in the past few decades has been on promoting P use and its efficient management in rainfed and irrigated agriculture. In the 1960s and 1970s, virtually no fertilizer was used in the region, with rapid increases in N and to a lesser extent P since then. The sharp transition from low-input traditional agriculture to conventional modern agriculture has particular implications for efficient P fertilizer use from the economic and environmental standpoints. This review seeks to present a broad overview of P in countries of the WANA region, which varies considerably with respect to economic development and the level of agricultural research, education and extension. It presents the background global considerations with respect to P supplies and use, as well the agricultural context for the region, including climate and cropping systems; it draws heavily on research on soils and soil-P chemistry from Spain, which though technically excluded from WANA, has much in common with the Mediterranean region, and highlights P research from Pakistan at the eastern fringes of WANA. It highlights the discrepancy in P use between developed and developing countries such as those of WANA. The review to some extent builds on extensive research carried out in Syria by the International Center for Agricultural Research in the Dry Areas (ICARDA), with secondary emphasis on countries of the region, many of which collaborate closely with ICARDA. The review covers the past three decades, highlighting progress in field trials on fertilizer use with the regions main crops in relation to rainfall, cropping systems, soil test levels, and efforts to identify P-efficient genotypes and enhance soil P fertility with mycorrhizae. Despite the many isolated, uncoordinated, and often-overlapping, and indeed conflicting, research efforts that have taken place in the region, we have attempted to show a gradual progression in knowledge of P in relation to soils and crops. Developments with regard to P, in the overall framework of agricultural research, have contributed to increased output in the WANA region. Much of the documented research has contributed to the global information on soils of arid and semi-arid regions. Despite achievements in applied research, poorly developed technology transfer systems and weak analytical facilities remain as stumbling blocks to the widespread dissemination of the accumulated knowledge on P use to farmers.

Evaluation of the CropSyst model for simulating the potential yield of cotton

Cotton produced in Uzbekistan has a low water and fertilizer use efficiency and yield is below its potential. To introduce improved production methods, knowledge is required on how the agro-ecosystem would respond to these alternatives. For this assessment, dynamic simulation models such as the crop-soil simulation model CropSyst are useful tools. CropSyst had never been applied to cotton, so it first was calibrated to the cotton variety Khorezm-127 grown under researcher-managed optimal conditions in the Khorezm region of Uzbekistan in 2005. The model performance was evaluated with a data set obtained in 2004 on two farmer-managed sites. Both data sets comprised in-situ measurements of leaf area index and aboveground biomass. In addition, the 2004 data set included the normalized difference vegetation index derived from satellite imagery of the two cotton fields, which provided estimations of leaf area index with a high temporal resolution. The calibrated optimum mean daily temperature for cotton growth was 25 °C., the specific leaf area 13.0 m2 kg−1, the leaf/stem partition coefficient 3.0, the biomass/transpiration coefficient 8.1 kg m−2 kPa m−2 and the radiation use efficiency 2.0 g MJ−1. Simulations matched 2005 data, achieving a root mean square error between simulated and observed leaf area index and aboveground biomass of 0.36 m2 m−2 and 0.97 Mg ha−1, respectively. The evaluation showed that early cotton growth and leaf area index development could be simulated with sufficient accuracy using CropSyst. However, final aboveground biomass was slightly overestimated by CropSyst, because some unaccounted plant stress at the sites diminished actual aboveground biomass, leading to a root means square error of around 2 Mg ha−1. Some characteristics of cotton, such as the indeterminate growth habit, could not be incorporated in detail in the model. However, these simplifications were compensated by various other advantages of CropSyst, such as the option to simulate crop-rotation or its generic crop growth routine that allows modelling of additional, undocumented crops. The availability of normalized difference vegetation index data with a high temporal and acceptable spatial resolution opened possibilities for a precise, in-expensive and resource-efficient way of model evaluation.