Climate change on crop diversification, biodiversity, soil fertility, weed, pest and microbes dynamics

                 

           The type of crop that can be grown is affected by changes in temperatures and the length of the growing season. Climate change could also modify the availability of water for production. Farmers in several countries including Canada, India, Kenya, Mozambique and Sri Lanka have already initiated diversification as a response to climate change. Government policy in Kenya to promote crop diversification has included the removal of subsidies for some crops, encouraging land-use zoning and introducing differential land tax systems

Crop diversification refers to the addition of new crops or cropping systems to agricultural production on a particular farm taking into account the different returns from value-added crops with complementary marketing opportunities.

Breeding new and improved crop varieties enhances the resistance of plants to a variety of stresses that could result from climate change. These potential stresses include water and heat stress, water salinity, water stress and the emergence of new pests. Varieties that are developed to resist these conditions will help to ensure that agricultural production can continue and even improve despite uncertainties about future impacts of climate change. Varieties with improved nutritional content can provide benefits for animals and humans alike, reducing vulnerability to illness and improving overall health.

The aim of crop diversification is to increase crop portfolio so that farmers are not dependent on a single crop to generate their income. When farmers only cultivate one crop type they are exposed to high risks in the event of unforeseen climate events that could severely impact agricultural production, such as emergence of pests and the sudden onset of frost or drought. Introducing a greater range of varieties also leads to diversification of agricultural production which can increase natural biodiversity, strengthening the ability of the agro-ecosystem to respond to these stresses, reducing the risk of total crop failure and also providing producers with alternative means of generating income.

With a diversified plot, the farmer increases the chances of dealing with the uncertainty and/or the changes created by climate change. This is because crops will respond to climate scenarios in different ways. Whereas the cold may affect one crop negatively, production in an alternative crop may increase.

Farmer experimentation using only native varieties can limit the range of benefits and responses that may be found amongst the materials being tested, although local adaptation and acceptance are ensured. At the same time, problems can with the introduction of exotic species that after being introduced turning into pests.

A limitation of crop diversification is that it may be difficult for farmers to achieve a high yield in terms of tons per hectare given that they have a greater range of crops to manage. In terms of commercial farming, access to national and international markets may be limited by a range of factors including government policy including subsidies, the price and supply of inputs, infrastructure for storage and transportation, amongst others. Farmers also face risk from poor economic returns if crops are not selected based on a market assessment. For example, drought tolerant crop varieties may fetch a low market price if there is not sufficient demand.

The main barrier to introducing new and improved crop varieties through farmer experimentation is the misconception that local species have low productivity. In the same vein, several communities in developing countries have lost ancient knowledge about resistant species.

In Malawi, Zambia, and Mozambique the majority of farmers currently adopt one of seven different cropping systems, based on a combination of four categories of crops: dominate staple (maize), alternative staples, legumes and cash crops.

Diversified cropping systems contribute to climate smart agricultural pillars Compared to maize mono-cropping, the majority of cropping systems have neutral to positive effects on smallholder productivity, though the magnitude of the impact varies.

For example, in Malawi maize-legume systems are associated with 17–38 percent increase in maize yields, compared with maize monocropping. More diverse systems likely enable farmers to capture a combination of agronomic benefits in the soil, such as through phosphorus enhancement and nitrogen fixation, from a range of crops, which contribute to improved yields of maize in addition to compaction of ill effects of climate change.

In terms of resilience, which is measured as a reduction in crop income variability, more diversified systems, particularly those that incorporate legumes, significantly reduce crop income variability compared with maize mono-cropping. However, no single system decreases significantly crop income volatility in all countries, suggesting that this impact depends strongly on the agronomic attributes of the specific crop grown in different countries and on associated market structures.

In Zambia, for example, households residing in villages with more private grain buyers are significantly more likely to move away from maize monocropping and adopt more diverse, commercially oriented systems, including systems that feature legumes and cash crops.

A gradual, continuing rise in atmospheric CO₂ concentration entailing increased photosynthetic rates and water-use efficiencies of vegetation and crops, hence increases in organic matter supplies to soils.

Minor increases in soil temperatures in the tropics and subtropics; moderate increases and extended periods in which soils are warm enough for microbial activity (warmer than about 5°C) in temperate and cold climates, parallel to the changes in air temperatures and vegetation zones.

Increases in amount and in variability of rainfall in the tropics; possible decrease in rainfall in a band in the subtropics poleward of the present deserts; minor increases in amount and variability in temperate and cold regions. Peak rainfall intensities could increase in several regions.

Effects of higher CO₂ on soil fertility and productivity

Higher atmospheric CO₂ concentration increases growth rates and water-use efficiency of crops and natural vegetation in so far as other factors do not become limiting. The higher temperature optima of some plants under increased CO₂ would tend to counteract adverse effects of temperature rise, such as increased nighttime respiration. The shortened growth cycle of a given species because of higher CO₂ and temperature would be compensated for in natural vegetation by adjustments in species composition or dominance. In agro-ecosystems the choice of longer-duration cultivars or changes in cropping pattern could eliminate unproductive periods that might arise because of the shorter growth cycle of the main crop.

Increased productivity is generally accompanied by more litter or crop residues, a greater total root mass and root exudation, increased mycorrhizal colonization and activity of other rhizosphere or soil micro-organisms, including symbiotic and root-zone N, fixers. The latter would have a positive effect on N supply to crops or vegetation.

The increased microbial and root activity in the soil would entail higher CO₂ partial pressure in soil air and CO₂ activity in soil water, hence increased rates of plant nutrient release (e.g., K, Mg, micronutrients) from weathering of soil minerals. Similarly, the mycorrhizal activity would lead to better phosphate uptake. These effects would be in synergy with better nutrient uptake by the more intensive root system due to higher atmospheric CO₂ concentration.

The greater microbial activity tends to increase the quantity of plant nutrients cycling through soil organisms. The increased production of root material (at similar temperatures) tends to raise soil organic matter content, which also entails the temporary immobilization and cycling of greater quantities of plant nutrients in the soil.

Higher C/N ratios in litter, reported by some workers under high CO₂ conditions, would entail slower decomposition and slower remobilization of the plant nutrients from the litter and uptake by the root mat, and would provide more time for incorporation into the soil by earthworms, termites, etc. Higher soil temperatures would counteract increases in 'stable' soil organic matter content but would further stimulate microbial activity.

Consequently rapid increase in soil organic matter dynamics and soil micro-organisms may cause temporary competition for plant nutrients. These temporary effects have on occasion been reported as negative factors affecting plant response to elevated CO₂. However, increased organic matter dynamics and microbial activity in soils are positive for the soil-plant system when CO₂ concentrations rise gradually over decades, as currently and in the recent past.

Increased microbial activity due to higher CO₂ concentration and temperature produces greater amounts of polysaccharides and other soil stabilizers. Increases in litter or crop residues, root mass and organic matter content tend to stimulate the activity of soil macro-fauna, including earthworms, with consequently improved infiltration rate and bypass flow by the greater number of stable biopores.

The greater stability and the faster infiltration increase the resilience of the soil against water erosion and loss of soil fertility. The increased proportion of bypass flow also decreases the nutrient loss by leaching during periods with excess rainfall. This refers to the available nutrients in the soil, including well-incorporated fertilizers or manure, but not to fertilizers broadcast on the soil surface. These are subject to loss by runoff or leaching.

In subtropical and other sub-humid or semi-arid areas, the increased productivity and water-use efficiency due to higher CO₂ would tend to increase ground cover, counteracting the effects of higher temperatures. If there would be locally much less rainfall and increasing intra and inter-annual variability, these could lead to less dry-matter production and hence, in due course, lower soil organic matter contents.

Periodic leaching during high-intensity rainfall with less standing vegetation could desalinize some soils in well-drained sites, cause increased runoff in others, and lead to soil salinization in depressional sites or where the groundwater table is high. Soils most resilient against the effects of such increasing aridity and rainfall variability would have a high structural stability and a strongly heterogeneous system of continuous macro-pores (the same as in the tropics); hence a rapid infiltration rate, as well as a large available water capacity and a deep groundwater table.

Higher temperatures, particularly in arid conditions, entail a higher evaporative demand. Where there is sufficient soil moisture, for example in irrigated areas, this could lead to soil salinization if land or farm water management, or irrigation scheduling or drainage are inadequate. On the other hand, recent experiments by the Salinity Laboratory, Riverside, California, point to increased salt tolerance of crops under high atmospheric CO₂ conditions.

In temperate climates, minor increases in rainfall totals would be expected to be largely taken up by increased evapo-transpiration of vegetation or crops at the expected higher temperatures, so that net hydrologic or chemical effects on the soils might be small.

The negative effect on soil organic matter contents of a temperature rise might be more than compensated by the greater organic matter supply from vegetation or crops growing more vigorously because of the higher photosynthesis, the greater potential evapo-transpiration and the higher water-use efficiency in a high-CO₂ atmosphere.

A rising sea level would tend to erode and move back existing coastlines. However, the extent to which this actually happens will depend on the elevation, the resistance of local coastal materials, the degree to which they are defended by sediments provided by river flow or long shore drift, the strength of long shore currents and storm waves, and on human interventions which might prevent or accelerate erosion.

In coastal lowlands which are insufficiently defended by sediment supply or embankments, tidal flooding by saline water will tend to penetrate further inland than at present, extending the area of perennially or seasonally saline soils. Where Rhizophora mangrove or Phragmites vegetation invades the area, that would over several decades lead to the formation of potential acid sulphate soils. Impedance of drainage from the land by a higher sea level and by the correspondingly higher levels of adjoining estuarine rivers and their levees, will also extend the area of perennially or seasonally reduced soils and increase normal inundation depths and durations in river and estuary basins and on levee back slopes.

Soil management measures designed to optimize the soil's sustained productive capacity would therefore be generally adequate to counteract any degradation of agricultural land by climate change.

Use an integrated plant nutrient management system to balance the input and off take of nutrients over a cropping cycle or over the years, while maintaining soil nutrient levels low enough to minimize losses and high enough to buffer occasional high demands.

An analogous philosophy, at lower levels of external inputs, could be formulated for extensive grazing land and production forest, whether planted or managed natural forest.

The anticipated impacts of climate change are warmer conditions, an increasing proportion of rainfall to occur from heavy falls, increasing occurrence of drought in many regions, increasing frequency of intense tropical cyclones, rising sea levels and frequency of extreme high seas (e.g., storm surges). All of these predicted impacts have direct relevance to coastal acid sulfate soils landscapes, through either exacerbating sulfide oxidation by drought, re-instating reductive geochemical processes or changing the export and mobilisation of contaminants. The interaction of specific land management factors such as man-made drainage will also have a significant role in how the predicted impacts of climate change affect these landscapes.

Understanding the potential impacts of climate change for coastal lowland acid sulfate soils is particularly important, given the utility of these areas for agriculture and urban communities, their unique capacity to cause extreme environmental degradation and their sensitivity to climatic factors such as temperature and hydrology and susceptibility to sea-level inundation.

Vulnerability of biodiversity to the impacts of climate change

The present global biota has been affected by fluctuating Pleistocene (last 1.8 million years) concentrations of atmospheric carbon dioxide, temperature, precipitation, and has coped through evolutionary changes, and the adoption of natural adaptive strategies. Such climate changes, however, occurred over an extended period of time in a landscape that was not as fragmented as it is today and with little or no additional pressure from human activities. Habitat fragmentation has confined many species to relatively small areas within their previous ranges, resulting in reduced genetic variability. Warming beyond the ceiling of temperatures reached during the Pleistocene will stress ecosystems and their biodiversity far beyond the levels imposed by the global climatic change that occurred in the recent evolutionary past.

Current rates and magnitude of species extinction far exceed normal background rates. Human activities have already resulted in the loss of biodiversity and thus may have affected goods and services crucial for human well-being. The rate and magnitude of climate change induced by increased greenhouse gases emissions has and will continue to affect biodiversity either directly or in combination with other drivers of change.

There is ample evidence that climate change affects biodiversity. According to the Millennium Ecosystem Assessment, climate change is likely to become one of the most significant drivers of biodiversity loss by the end of the century. Climate change is already forcing biodiversity to adapt either through shifting habitat, changing life cycles, or the development of new physical traits.

Conserving natural terrestrial, freshwater and marine ecosystems and restoring degraded ecosystems (including their genetic and species diversity) is essential for the overall goals of both the Convention on Biological Diversity and the United Nations Framework Convention on Climate Change because ecosystems play a key role in the global carbon cycle and in adapting to climate change, while also providing a wide range of ecosystem services that are essential for human well-being and the achievement of the Millennium Development Goals.

Biodiversity can support efforts to reduce the negative effects of climate change. Conserved or restored habitats can remove carbon dioxide from the atmosphere, thus helping to address climate change by storing carbon (for example, reducing emissions from deforestation and forest degradation). Moreover, conserving in-tact ecosystems, such as mangroves, for example, can help reduce the disastrous impacts of climate change such as flooding and storm surges. Conservation of agrobiodiversity to provide specific gene pools for crop and livestock adaptation to climate change.

The rising levels of CO₂ and temperatures are having direct effect on pests and diseases in crops. Elevated CO₂ can increase levels of simple sugars in leaves and lower their nitrogen content. These can increase the damage caused by many insects, who will consume more leaves to meet their metabolic requirements of nitrogen. Thus, any attack will be more severe.

Higher temperatures from global warming, mainly due to elevated CO₂, will mean that more numbers of pests will survive the winter season. Elevated CO₂ will help in easier over-wintering of pathogens while higher temperatures will favour thermophilic fungi . Higher temperatures will lead to a poleward spread of many pests and diseases in both hemispheres. This will lead to more attacks over longer periods in the temperate climatic zone

Other possible effects of climate change need to be taken into account. On one hand, warmer temperature lowers the effectiveness of some pesticides but on the other hand, it favours insect carriers of many disease pathogens and natural enemies of pests and diseases. Thus, depending on the pest or pathogen, elevated CO₂ may act in a synergic or opposing manner with higher temperatures. Results of such interactions are difficult to be anticipated. Thus, one is obliged to wait for visual signs of appearance of a pest or disease for initiating action.

Elevated CO₂ levels and higher temperatures will keep changing the composition and duration of infective stages of pests and diseases. The current agromet models for anticipation and control of crop pests and diseases will thus be ineffective.

As mentioned above, elevated carbon dioxide and higher temperature may act in a synergic or opposing manner depending on the pest or pathogen concerned. The result of these changes cannot be foreseen as yet and waiting for visual appearance of a pest or disease to initiate action, is the only remaining option. Organising manual surveys to cover all major crop pests and diseases will be a very costly and nearly impossible.

The increase in temperatures will be more at night than during daytime. Higher nocturnal temperature will reduce the duration of Leaf-Wetness and result in lesser disease incidence. Biological control of a pest or disease through introduction of their natural enemies from other regions will become more effective. Warm temperatures will favour their quick establishment and development.

Global warming of 2^O C above pre-industrial levels – which is the limit set by the Paris Agreement – could cause pest-related yield losses from wheat, rice and maize to increase by 46%, 19% and 31%, respectively.

Each additional degree of temperature rise could cause yield losses from insect pests to increase by a further 10-25%.

Losses from pest infestation are likely to be largest in China, the US and France – three of the world’s most important grain producers, according to the findings.

At present, around 10-16% of global crop production is lost to pests – including insects, fungi and bacteria.

Thousands of insect species are known to threaten food production. One of the most well-known pests, the desert locust, feeds on a wide range of crops – including rice, maize and sugarcane – and can swarm and strip a crop field within an hour.

Other insects, such as the western corn rootworm, target specific crops. The rootworm, for example, feeds on maize during both its larval and adult beetle life stages.

The new study, published in Science, explores how climate change could alter the activity of 38 of the world’s most-studied insect crop pests.

Climate change could increase the activity of insect pests in two ways. First, rising temperatures boost the rate at which insects can digest food – causing them to demolish crops at a faster rate.  Second, in temperate regions, warming temperatures could cause insects – which are ectothermic, or “cold-blooded” –  to become more active and, thus, more able to reproduce.

Rice, on the other hand, is grown mostly in tropical regions – where temperatures are already optimal for insect reproduction. Further temperature increases are therefore likely to cause small declines in insect numbers, the research finds, leading to an overall smaller effect on yield losses.

Soil microorganisms regulate nutrient transformations, provide plants with nutrients, allow co‐existence among neighbors, and control plant populations, changes in soil microorganism‐plant interactions could have significant ramifications for plant community composition and ecosystem function.

Climate change impacts on plant‐microbial interactions

Soil biota may be poor dispersers, therefore they may respond to climate change at a different rate than plants. Some range expanding plant species are better defended against aboveground herbivores and/or develop less pathogenic activity in their soils compared to their related natives in the new range. If plants that successfully establish in new ranges have higher induced levels of plant defense compounds such as polyphenols, then litter input quality will decline and the decomposer community will shift in composition or activity.