Green leaf biomass of Tithonia diversifolia is high in nutrients and recognised as a potential source of nutrients for crops. We conducted a field survey in western Kenya to determine the variation in leaf nutrient concentrations in tithonia grown in naturalised hedges and agricultural fields, and to examine whether leaf nutrient concentrations were related to soil nutrient status. Leaf P and K concentrations were higher in naturalised hedges (3.2 g P kg1 and 35 g K kg1) than in unfertilised fields (2.2 g P kg1 and 23 g K kg1). The critical level of 2.5 g P kg1 for net P mineralisation was exceeded by > 90% of the leaves from hedges, but by only 14% from unfertilised fields. Leaf P and K concentration increased linearly with increasing natural logarithm of anion resin extractable soil P and exchangeable soil K, respectively. However, at the same levels of soil available P and K, field-grown tithonia consistently produced lower leaf P and K concentrations than that grown in hedges. This study indicates that biomass from tithonia planted on nutrient-depleted soils would be a less effective source of P and K, via biomass transfer, than tithonia from naturalised hedges.
Tag: nutrient cycling
Species screening for short-term planted fallows in the highlands of western Kenya
Short-term improved fallow technology, which is characterised by deliberate planting of fast growing N2 fixing legumes species in rotation with crops is currently being promoted for soil fertility replenishment in the small holder farms in the tropics. Recent research and extension efforts on this technology have mainly focused on a narrow range of species. There is a need to evaluate more alternative species in order to diversify the options available to farmers and hence reduce the risks of over dependence on fewer species. We evaluated twenty-two shrubby and herbaceous species for their site adaptability, biomass and nutrient accumulation, biomass quality and maize yield response to soil incorporated plant biomass after the fallow (six and twelve months) in three different field experiments on a Kandiudalfic Eutrudox in western Kenya. Species which yielded large amounts ofthe most biomass N adequate for two to three maize crops were Sesbania sesban, Tephrosia vogelii, Tephrosia candida, Crotalaria grahamiana, Dodonea viscosa, Colopogonium mucunoides, Desmondium uncinatum, Glycine wightii and Macroptilium atropurpureum. Most fallow species tested recycled 10%) and polyphenol (>2%) concentrations. were found only in the shrubby species, and the (Ppolyphenol + lignin ): N ratio varied widely (0.3–5) amongst the species. evaluated. Maize yield increased by two-fold in the first season following the fallow phase compared with continuous maize for most species. Results suggest that there are a wide variety of legumes that could be used for use in improved fallow technologies aimed at ameliorating nutrient degraded soils and subsequently enhancing crop yields.
Soil biological dynamics in slash-and-burn agriculture
Studies of shifting cultivation and other slash-and-burn systems over the past 30 years have basically confirmed the conceptual model of carbon and nutrient cycling put forth by Nye and Greenland. The model stresses that soil biological processes should not be viewed in isolation but as an integral part of the system. There has been some progress in refining certain aspects of the model but most studies have merely provided more numbers for specific fluxes or pools in the cycle rather than the entire cycle. While these studies have reinforced the model of Nye and Greenland, they have not added much to understanding the controls or improving the predictive capacity that would allow improved management of slash-and-burn systems. Future studies should concentrate on controlling and manipulating certain components in the system and looking at the resulting changes in the other pools and processes in the system. Results from these types of studies can be combined with system models for simulating and comparing different management strategies.
The science and practice of short-term improved fallows: symposium synthesis and recommendations
This paper summarizes the reports submitted by working groups who met during the International Symposium on the Science and Practice of Short-term Improved Fallows held in Lilongwe, Malawi from 11–15 March 1997. The four working groups focused their discussions on the themes around which the symposium was structured, namely (1) traditional fallows and indigenous knowledge, (2) case studies on research and development of improved fallows, (3) how and where fallows work, and (4) dissemination mechanisms and policy requirements for adoption. Through their deliberations, each group provided a synthesis of key lessons learned and recommendations for future research and action.
Ecosystem fertility and fallow function in the humid and subhumid tropics
The regeneration of natural vegetation (fallowing) is a traditional practice for restoring fertility of agricultural land in many parts in the tropics. As a result of increasing human population and insufficient fertilizer inputs, the ecosystem fertility functions of traditional fallows must now be improved upon via the use of managed fallows. Interactions between vegetation and soil determine nutrient losses and gains in crop—fallow systems and are influenced by fallow species, patterns and rates of biomass allocation, and crop and fallow management. Nutrient losses occur through offtake in crop harvests during the cropping phase and through leaching, runoff, and erosion in the cropping phase and the initial stage of fallows $#x2014; when nutrient availability exceeds nutrient demand by vegetation. Gains in nutrient stocks in later stages of fallow are generally more rapid on soils with high than low base status due to greater quantities of weatherable minerals and lack of constraints to N2 fixation, deep rooting, and retrieval of subsoil nutrients by fallow vegetation. On low base status soils (exchangeable Ca 5 yr) are needed for recovery of cation stocks and crop performance on low base status soils. On both soils, however, residual benefits of fallows on crop yields usually last less than three crops.
Microplastics as an emerging threat to plant and soil health in agroecosystems
Microplastics (MPs, <5 mm in diameter) have been widely recognized as a critical environmental issue due to their extensive use and low degradation rate. Based on current evidence, our aim is to evaluate whether MPs represent an emerging threat to plant-soil health in agroecosystems. We assess the ecological risks to plant-microbe-soil interactions associated with MPs and discuss the consequences of MPs on soil carbon (C), nutrient cycling, as well as greenhouse gas emissions in agroecosystems. We also identify knowledge gaps and give suggestions for future research. We conclude that MPs can alter a range of key soil biogeochemical processes by changing its properties, forming specific microbial hotspots, resulting in multiple effects on microbial activities and functions. Mixed effects of MPs on plant growth and performance can be explained by the direct toxicity of MPs or the indirect alteration in soil physical structures and microbial communities (i.e. symbiotic arbuscular mycorrhizal fungi). Because of the diverse nature of MPs found in soils, in terms of polymer type, shape and size, we also see differing effects on soil organic matter (SOM) decomposition, nutrient cycling, and greenhouse gases production. Importantly, increased bioavailable C from the decomposition of biodegradable MPs, which enhances microbial and enzymatic activities, potentially accelerates SOM mineralization and increases nutrient competition between plant and microbes. Thus, biodegradable MPs appear to pose a greater risk to plant growth compared to petroleum-based MPs. Although MPs may confer some benefits in agroecosystems (e.g. enhanced soil structure, aeration), it is thought that these will be far outweighed by the potential disbenefits.
Litterfall seasonal dynamics and leaf-litter turnover in cocoa agroforests established on past forest lands or savannah
Nutrient cycling in cocoa agroforestry systems (cAFS) is complex and poorly understood. To better understand the mass flow of carbon and nutrients into the soil sub-system under various contexts we quantified the mass flow of litterfall, its composition and seasonal variations in different agroforestry systems in Bokito (Central Cameroon). We studied litterfall dynamics and in situ leaf-litter cycling of cAFS established on past forest lands (F-cAFS) and savannah (S-cAFS). We also studied the decomposition of cocoa and associated tree leaf-litter in litterbags. Local secondary semi-deciduous forests were included as control. Annual litterfall in full-grown cAFS (> 15 years old) was high (9.4 Mg ha−1 y−1) and represented ca. 67% of litterfall in control forests. In full-grown cAFS, associated tree leaf-litter contributed to litterfall the most and ranged between 60 and 70% of the total amount recorded (6.3 Mg ha−1 y−1). The quantities and dynamics of the litter components monitored were similar in full-grown S- and F-cAFS. The microclimate was best buffered in forests and least buffered in young S-cAFS but could not be linked to leaf-litter decomposition. Forest leaf litterfall was higher and tended to cycle faster than total leaf-litter of cAFS, whose decomposition appeared limited by cocoa leaf-litter quality. Our study underlines (i) the critical contribution of associated trees to the nutrient cycle of agroecosystems established on poor soils and, (ii) the ability of farmers to channel associated tree communities towards similar functioning despite different past land-uses.
Unraveling consequences of soil micro- and nano-plastic pollution on soil-plant system: Implications for nitrogen (N) cycling and soil microbial activity
Micro- and nano-plastics have widely been recognized as major global environmental problem due to its widespread use and inadequate waste management. The emergence of these plastic pollutants in agroecosystem is a legitimate ecotoxicological concerns for food web exchanges. In agriculture, micro/nano plastics are originated from a variety of different agricultural management practices, such as the use of compost, sewage sludge and mulching. A range of soil properties and plant traits are affected by their presence. With the increase of plastic debris, these pollutant materials have now begun to demonstrate serious implications for key soil ecosystem functions, such as soil microbial activity and nutrient cycling. Nitrogen (N) cycle is key predictor of ecological stability and management in terrestrial ecosystem. In this review, we evaluate ecological risks associated with micro-nano plastic for soil-plant system. We also discuss the consequence of plastic pollutants, either positive or negative, on soil microbial activities. In addition, we systematically summarize both direct and hypothesized implications for N cycling in agroecosystem. We conclude that soil N transformation had showed varied effects resulting from different types and sizes of plastic polymers present in soil. While mixed effects of microplastic pollution on plant growth and yield have been observed, biodegradable plastics have appeared to pose greater risk for plant growth compared to chemical plastic polymers.
Do Added Microplastics, Native Soil Properties, and Prevailing Climatic Conditions Have Consequences for Carbon and Nitrogen Contents in Soil? A Global Data Synthesis of Pot and Greenhouse Studies
Microplastics threaten soil ecosystems, strongly influencing carbon (C) and nitrogen (N) contents. Interactions between microplastic properties and climatic and edaphic factors are poorly understood. We conducted a meta-analysis to assess the interactive effects of microplastic properties (type, shape, size, and content), native soil properties (texture, pH, and dissolved organic carbon (DOC)) and climatic factors (precipitation and temperature) on C and N contents in soil. We found that low-density polyethylene reduced total nitrogen (TN) content, whereas biodegradable polylactic acid led to a decrease in soil organic carbon (SOC). Microplastic fragments especially depleted TN, reducing aggregate stability, increasing N-mineralization and leaching, and consequently increasing the soil C/N ratio. Microplastic size affected outcomes; those <200 μm reduced both TN and SOC contents. Mineralization-induced nutrient losses were greatest at microplastic contents between 1 and 2.5% of soil weight. Sandy soils suffered the highest microplastic contamination-induced nutrient depletion. Alkaline soils showed the greatest SOC depletion, suggesting high SOC degradability. In low-DOC soils, microplastic contamination caused 2-fold greater TN depletion than in soils with high DOC. Sites with high precipitation and temperature had greatest decrease in TN and SOC contents. In conclusion, there are complex interactions determining microplastic impacts on soil health. Microplastic contamination always risks soil C and N depletion, but the severity depends on microplastic characteristics, native soil properties, and climatic conditions, with potential exacerbation by greenhouse emission-induced climate change.