Soil and water management is an essential element in food security, agriculture sector growth and sustainable land management of sub-Saharan Africa (SSA). The increased land degradation and declining fertility of SSA soils contribute to food insecurity and poverty. Previously, agroforestry researchers tended to focus mostly on soil nutrient replenishment as being solely responsible for post-fallow crop yield dynamics. Missing from many studies on soil fertility issues is the recognition of the important role of soil physical properties in agricultural productivity. However, many factors affect soil fertility and some agroforestry measures taken to correct soil nutrient deficiencies can also produce desirable soil physical effects. We hypothesized that planted tree fallows can potentially increase soil N status and improve soil physical properties, thus increasing subsequent crop yields. Field studies were conducted on infertile sandy clay loams at Msekera and Kagoro, Zambia, to determine the effect of contrasting fallows (natural fallow, planted non-coppicing and coppicing tree fallows) and no-tree no-fallow [maize (Zea mays L.) with and without fertilizer] on soil fertility and maize yields. This study attempted to address agricultural productivity by viewing soil fertility in terms of both chemical and physical properties. Hence, this report discusses the implications of improving the nutrient status of soils without correcting soil physical constraints. Data from both tree-(agroforestry) and non-tree-based systems have been used to illustrate important physical and chemical changes that occur in soils as a consequence of varying management regimes or cropping systems. Such data show that the concept of soil productivity refers to more than replacement of the lost nutrients. Other aspects include soil structure, soil water retention, water storage, infiltration and soil penetration resistance. The results imply that standard inputs such as mineral or organic fertilizers can maintain only some elements of soil productivity. Therefore, a broader view that incorporates the role of soil physical properties and water in influencing productivity is appropriate. This research does not attempt to provide a comprehensive treatment of all aspects of soil fertility in agroforestry systems. For example, it does not address the role of soil biota diversity in soil productivity.
Tag: nitrogen
Rooting depth, synchronization, synlocalization and N-use efficiency under humid tropical conditions
A model, is presented with which calculations on the nitrogen balance of a soil can be performed. The processes considered include leaching, transformation of mineral and organic N, adsorption, and plant uptake. Model calculations on the nitrogen balance for conditions of continuous leaching during the growing season in the humid tropics show a reasonable agreement between N uptake as predicted on the basis of observed root distribution and actually measured uptake. Practical possibilities for increasing nutrient use efficiency through better synchronization and synlocalization of nutrient supply in relation to nutrient demand by the crop are discussed.
Replenishing soil fertility in Africa: Proceedings of an International Symposium by Division A-6 (International Agronomy) and S-4 (Soil Fertility and Plant Nutrition), and ICRAF, held at Indianapolis, Indiana 6 November 1996
Soil-fertility depletion in smallholder farms is the fundamental biophysical root cause for declining per capita food production in sub-Saharan Africa. An average of 660 kg N ha-1, 75 kg P ha-1, and 450 kg K ha-1 has been lost during the last 30 yr from about 200 million ha of cultivated land in 37 African countries. We propose an alternative approach, the replenishment of soil fertility as an investment in natural resource capital. This approach combines basic principles of soil science with environmental economics. Combinations of P fertilizers and organic inputs can replenish soil N and P nutrient stocks in Africa and restore service flows to near original levels. Phosphorus replenishment strategies are mainly mineral-fertilizer based, with biological supplementation. Nitrogen replenishment strategies are mainly biologically based with mineral-fertilizer supplemen-tation. Africa has ample phosphate rock (PR) deposits that can be either used directly or processed to reverse P depletion. Decomposing organic inputs may facilitate the use of PR in P-depleted soils. Leguminous tree fallows and herbaceous cover crops grown in situ play a major role in N capture and internal cycling in ways compatible with farmer con-straints. Soil-fertility replenishment was found profitable in three case studies, but small-holder farmers lack the capital and access to credit to make the initial investment. A cost-shared initial capital investment to purchase P fertilizer and germplasm for growth of organic inputs combined with effective microcredit for recurring costs such as fertilizers and hybrid seed is seen as the way forward.
Symbiotic Nitrogen Fixation in Soil Contaminated with the Veterinary Antibiotics Oxytetracycline and Sulfamethazine
Veterinary and growth‐promoting antibiotics are widely used in animal husbandry and accumulate in manure‐fertilized soils. However, the impact of these antibiotics on symbiotic nitrogen fixation is poorly understood. We investigated the effect of the veterinary antibiotics oxytetracycline and sulfamethazine, and a combination of both, on nitrogen fixation in alfalfa (Medicago sativa L.) inoculated with Sinorhizobium meliloti. In a pot experiment, M. sativa was grown in soils fertilized with fresh manure that contained environmentally relevant antibiotic concentrations (0.2, 2, and 200 mg kg−1). Nodulation, nitrogen fixation, and nutrient concentrations were determined in the alfalfa plants and soils after 12 wk. Compared with the antibiotic‐free control, symbiotic nitrogen fixation increased significantly in soils mixed with manure containing 2 and 200 mg kg−1 oxytetracycline (20.1 and 20.8% increase, respectively) and a mixture of 200 mg kg−1 oxytetracycline and sulfamethazine (12.4% increase). The measured plant‐ and soil‐related parameters failed to explain the observed increase in nitrogen fixation. However, using concentration levels that accurately reflect common agricultural practices, we obtained results that directly contradict other experiments conducted under unrealistically high antibiotic concentrations.
Nitrogen turnover and N2O/N2 ratio of three contrasting tropical soils amended with biochar
Biochar has been reported to reduce emission of nitrous oxide (N2O) from soils, but the mechanisms responsible remain fragmentary. For example, it is unclear how biochar effects on N2O emissions are mediated through biochar effects on soil gross N turnover rates. Hence, we conducted an incubation study with three contrasting agricultural soils from Kenya (an Acrisol cultivated for 10-years (Acrisol10); an Acrisol cultivated for over 100-years (Acrisol100); a Ferralsol cultivated for over 100 years (Ferralsol)). The soils were amended with biochar at either 2% or 4% w/w. The 15N pool dilution technique was used to quantify gross N mineralization and nitrification and microbial consumption of extractable N over a 20-day incubation period at 25 °C and 70% water holding capacity of the soil, accompanied by N2O emissions measurements. Direct measurements of N2 emissions were conducted using the helium gas flow soil core method. N2O emissions varied across soils with higher emissions in Acrisols than in Ferralsols. Addition of 2% biochar reduced N2O emissions in all soils by 53 to 78% with no significant further reduction induced by addition at 4%. Biochar effects on soil nitrate concentrations were highly variable across soils, ranging from a reduction, no effect and an increase. Biochar addition stimulated gross N mineralization in Acrisol-10 and Acrisol-100 soils at both addition rates with no effect observed for the Ferralsol. In contrast, gross nitrification was stimulated in only one soil but only at a 4% application rate. Also, biochar effects on increased NH4+ immobilization and NO3−consumption strongly varied across the three investigated soils. The variable and bidirectional biochar effects on gross N turnover in conjunction with the unambiguous and consistent reduction of N2O emissions suggested that the inhibiting effect of biochar on soil N2O emission seemed to be decoupled from gross microbial N turnover processes. With biochar application, N2 emissions were about an order of magnitude higher for Acrisol-10 soils compared to Acrisol-100 and Ferralsol-100 soils. Our N2O and N2 flux data thus support an explanation of direct promotion of gross N2O reduction by biochar rather than effects on soil extractable N dynamics. Effects of biochar on soil extractable N and gross N turnover, however, might be highly variable across different soils as found here for three typical agricultural soils of Kenya.
Fallow and sesbania effects on soil nitrogen dynamics in lowland rice-based cropping systems
Vast areas of rice-growing (Oryza sativa L.) lowlands in Asia are fallowed or cropped with non-rice crops for part of the year. Nitrate can accumulate during the fallow or non-rice crop, but this nitrate can be lost upon flooding for rice production. To determine fallow and green manure crop effects on soil nitrate and ammonium dynamics in lowland riceland, a 2-yr field study was conducted in the Philippines. Treatments before wet season rice were (i) Sesbania rostrata grown for either 45 or 60 d, (ii) weedy fallow, and (iii) weed-free fallow. Sesbania rostrata was sown with irrigation in late April-early May, rains started in early (1989) or mid-May (1990). Weeds and S. rostrata were incorporated after soil flooding on 23 June. Rains increased soil water-filled pore space to above 0.75 mL mL1 between mid-May and soil flooding. Weeds and S. rostrata assimilated soil nitrate, as evidenced by lower (P < 0.05) nitrate in those treatments than in the weed-free fallow. The decrease in soil nitrate in the weedfree fallow from 24 April to before soil flooding (15 kg N ha1) was apparently due to denitrification or leaching; additional nitrate (19 kg N ha1 in 1990) disappeared after soil flooding. Ammonium-N was rapidly released from incorporated weeds and S. rostrata. It reached a maximum by 36 d after incorporation, which correlated (r = 0.95) with N accumulation by rice at 45 d after transplanting. Results suggest that weeds and crops before rice can reduce soil N loss by assimilating nitrate-N and then cycling this N through incorporated plant residues back to the soil where it is rapidly mineralized and used by rice.
Nitrogen dynamics of grain legume-weedy fallow-flooded rice sequences in the tropics
Dry-season (DS) grain legume-weedy fallow-wet-season (WS) flooded rice is a common cropping sequence in the rainfed lowlands of tropical Asia. To better manage N in this cropping system, we need to understand N dynamics and balances as influenced by the aerobic-anaerobic soil aeration sequence, legume cropping, biological N2 fixation (BNF), and recycling of legume residues. To understand N dynamics under a range of N derived from BNF (15N-estimated), harvested in pods and left in residues, we conducted a 2-yr experiment on a Philippine Alfisol using cowpea [Vigna unguiculata (L.) Walp.], mungbean [V. radiata (L.) Wilcz.], nodulating and nonnodulating soybean [Glycine max (L.) Merr.], and weeds. The main portion of soil mineral N (0 to 60 cm) was NO3 in the dry season and NH4 in the wet season. The sum of soil NO3 and soil N uptake at legume harvest exceeded the decrease in soil NO3 from legume seeding to harvest by 81 kg ha -1, indicating the continued production and legume uptake of soil NO3. The large differences in total N of legumes (46 to 238 kg N ha-1), however, were associated with differences in N derived from BNF (0 to 176 kg N ha-1). When pod N was excluded, legume N balance was, in most cases, negative. The average soil N depletion was 40 kg ha-1 from nonnodulating soybean, compared with 8 kg ha-1 from N2-fixing legumes. In terms of WS rice grain and N yields, legume cropping did not differ from weedy fallowing, despite greater (by up to 46 kg N
Water and nitrogen dynamics in rotational woodlots of five species in western Tanzania
The objective of this study was to compare the effects of woodlots of five tree species, continuous maize (Zea mays L.) and natural fallow on soil water and nitrogen dynamics in western Tanzania. The tree species evaluated were Acacia crassicarpa (A. Cunn. ex Benth.), Acacia julifera (Berth.), Acacia leptocarpa (A. Cunn. ex Benth), Leucaena pallida (Britton and Rose), and Senna siamea (Lamarck) Irwin & Barneby). The field experiment was established in November 1996 in a completely randomized block design replicated three times. Maize was intercropped between the trees during the first three years after planting and thereafter the trees were allowed to grow as pure woodlots for another two years. Transpiration by the trees was monitored when they were 3 years old using sap flow gauges. Soil water content was measured using the neutron probe approach between November 1999 and March 2001. Soil inorganic N profiles were measured when the trees were four years old in all treatments. The results indicated that the trees transpired more water than natural fallow vegetation during the dry season. The difference was apparent at a depth of 35 cm soil, but was more pronounced in deeper horizons. The water content in the entire soil profile under woodlots and natural fallow during the dry period was 0.01 to 0.06 cm3 cm3 lower than in the annual cropped plots. This pattern was reversed after rainfall, when woodlots of A. crassicarpa, A. leptocarpa, A. julifera, S. siamea and L. pallida contained greater quantity of stored water than natural fallow or continuous maize by as much as 0.00 to 0.02, 0.01 to 0.04, 0.01 to 0.04, 0.01 to 0.03 and 0.00 to 0.02 cm3 cm3, respectively. Natural fallow plots contained the lowest quantity of stored water within the entire profile during this period. Transpiration was greatest in A. crassicarpa and lowest in L. pallida. All tree species examined were `scavengers’ of N and retrieved inorganic N from soil horizons up to 2-m depth and increased its concentration close to their trunks. This study has provided evidence in semi-arid environments that woodlots can effectively retrieve subsoil N and store more soil water after rains than natural fallow and bare soil.
Decomposition and nitrogen release patterns of Paraserianthes falcataria tree residues under controlled incubation
Tree legumes can serve as nitrogen (N) source for cereals in resource poor farms where chemical fertilizer is financially unaffordable. Despite the increasing importance of Paraserianthes falcataria in tropical agroforestry systems of Southeast Asia, little information is available on the decomposition and N release patterns of P. falcataria. Quality of P. falcataria roots and leaves, as individual components and as a mixture, was determined before incubating in an 15N labeled acidic Ultisol under controlled laboratory conditions. Decomposition was monitored as CO2 evolution and inorganic N released over time. The aim was to determine inorganic soil N and pH dynamics as affected by residue quality. Residue quality assessment based on (Polyphenol + Lignin): N was in the order of P. falcataria leaves > P. falcataria mixture of leaves and roots > P. falcataria roots. The same order was observed for nitrogen and carbon mineralization rate (P <0.05), indicating that mixing of residues of varying quality would provide a means of strategically modifying nutrient release. P. falcataria leaves and the mixture of leaves and roots significantly (P<0.05) mitigated soil acidity while P. falcataria roots alone did not.
Nitrogen and phosphorus in African soils – what role for agroforesty?
The article looks at the role of agroforestry in nutrient cycling and focuses on the two main nutrients that limit production, namely nitrogen and phosphorus in a small-holder production system based on maize in Africa.