Although intensive agriculture is necessary to sustain the world's growing
population, accelerated soil erosion contributes to a decrease in the
environmental health of ecosystems at local, regional and global scales.
Reversing the process of land degradation using vegetative measures is of
utmost importance in such ecosystems. The present study critically analyzes
the effect of grasses in reversing the process of land degradation using a
systematic review. The collected information was segregated under three
different land use and land management situations. Meta-analysis was applied
to test the hypothesis that the use of grasses reduces runoff and soil erosion.
The effect of grasses was deduced for grass strip and in combination with
physical structures. Similarly, the effects of grasses were analyzed in
degraded pasture lands. The overall result of the meta-analysis showed that
infiltration capacity increased approximately 2-fold after planting
grasses across the slopes in agricultural fields. Grazing land management
through a cut-and-carry system increased conservation efficiencies by 42
and 63 % with respect to reduction in runoff and erosion, respectively.
Considering the comprehensive performance index (CPI), it has been observed that
hybrid Napier (
Water erosion is the main cause of land degradation, affecting an area of about 2 billion ha throughout the world, with the largest part in tropics, and affecting the two most important natural resources, namely soil and water (Mandal and Sharda, 2011a; De Oliveria et al., 2010; Keesstra et al., 2014; Novara et al., 2011, 2016; Seutloali and Beckedahl, 2015). Worldwide loss of water and sediment due to soil erosion is a major environmental threat (Prosdocimi et al., 2016; Pimentel, 1993). Soil erosion is accelerated due to high rainfall intensities (Keesstra et al., 2016), steep slopes (Beskow et al., 2009) and the fragile nature of topsoil (Lal, 1998; Rodrigo Comino et al., 2016; Ochoa et al., 2016). Many parts of the tropics in India have high annual rainfall confined to only 4 to 5 months (June–September). During the 7–8-month dry period, scarcity of water causes a severe shortage of fodder in farmlands, which leads to an increase in grazing pressure on forest and community lands. Nearly a third of the fodder requirement in India is met through forest resources in the form of grazing and cut fodder (MoEF, 1999). The process of land degradation in croplands and grasslands has been accelerated mainly by inappropriate land use (Nearing et al., 2005; Mandal et al., 2010) and mismanagement (Kagabo et al., 2013).
Generally, soil conservation planning requires knowledge of soil loss tolerance values, which show the higher limit of soil erosion rate that can be allowed without long-term land degradation (Jha et al., 2009). Strategies to reverse land degradation are critical since soil is a non-renewable resource (Mandal and Sharda, 2011b; Mandal et al., 2010). Soil erosion rates more than tolerance values are considered unacceptable (Mandal and Sharda, 2013), which leads to irreversible land degradation and need to be reduced through appropriate soil conservation measures (SCMs) (Biswas et al., 2015) The physical structures to check soil erosion are proven effective but are cost-intensive. Biological methods of soil and water conservation, especially grass-based methods, have been reported to be very cost-effective and suitable for sloppy lands. Perennial grasses provide ground cover throughout the year and help in reducing runoff and soil loss when used as barriers along the contour, particularly in hill slopes (Dhruvanarayana and Rambabu, 1983). Grasses are the key component in many ecosystems of the world (Parras-Alcántara et al., 2015; Hu et al., 2016; Mekonnen et al., 2016).
Grass species, in particular, have tremendous potentialities in soil conservation as grass roots have a great binding influence on soil particles (Novara et al., 2013; Ola et al., 2015). Due to resource scarcity and multiple competing enterprises that characterize most farming situations of rural India, farmers often lack the adequate resources to invest in physical soil conservation structures. Thus, the usefulness of grasses as a vegetative barrier is an alternative to the physical soil structures. Basically, these contour vegetative barriers/grass filter strips help in reducing soil erosion by acting as porous barriers which subsequently slow down the flow of runoff (Anigma, 2002; Mutegi et al., 2008).
The hilly region of India is characterized by geological fragility, land
marginality and vulnerability (Mandal and Sharda, 2013). The croplands in
sloppy areas suffer from excessive soil erosion and erosion-induced nutrient
depletion. Soil erosion in these areas ranges between
20 and 40 Mg ha
The grasslands in the middle and lower Himalayas are generally in the most neglected state with low productivity. In this predominantly grazing region, excessive reliance on animal husbandry under a growing population has exerted great pressure on the land. In tropical India, an average of 42 animals graze on a hectare of land compared to maximum threshold level of 5 animals (Sahay, 1999). Raising and maintenance of perennial grasses on degraded soils has been suggested as a means to improve soil quality and sequester carbon in the soil. Several studies have shown that the inclusion of grasses in the agricultural landscape often improves the productivity of system while providing opportunities to create carbon (C) sinks (Ghosh et al., 2009; Cogle et al., 2011; Huang et al., 2010; Mutegi et al., 2008). Soils typically account for 70–90 % of the total carbon sequestered in a grassland ecosystem (Batjes, 2001).
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In India most of the studies on the role of grasses as vegetative/filter strips have been done in isolation with fewer slope categories and with limited objectives restricted to soil erosion (Njoroge and Rao, 1994). Similarly, studies on grazing land management are also very scarce. We present here an analysis of the potential of grasses for reversing land degradations for which the meta-analysis was carried out. The objective of this study is to determine the effect of grasses in arresting soil loss; runoff, moisture conservation and carbon buildup in soils. Based on such information, conclusions regarding reversing land degradation through grasses can be drawn wherever similar land conditions are known.
Information on the usefulness of grasses in soil and water conservation was collected from published literature (Table 1a and b). Keeping in mind the role of grasses for arresting soil loss and runoff, all data were reoriented under three different categories, namely (i) the role of grasses as a vegetative barrier, (ii) the complementary role of grasses with physical soil structures and (iii) management of grazing lands. A total of 83 studies comprising 19 different sites in varied agro-climatic region were included in the data set for the analysis (Table 1a and b). Fifty-four of these studies were related to contour grass barrier (CGB), 12 were related to grazing and 17 were related to complementary role of grasses.
Meta-analysis was applied to test the hypotheses about role of grasses in reducing soil erosion by combining data from several experiments. The technique has been extensively used in natural resource management studies (Ilstect et al., 2007, Poeplau and Don, 2015; Osenberg et al., 1999).
Various attributes and normalized scores used for calculating CPI for different vegetative barriers.
We aim to synthesize and discuss the past scientific studies pertaining to
the effect of grasses in arable and non-arable lands on one of the key
determining soil processes, namely reduction in soil and water losses and
enhancement of infiltration. In order to produce a combined data set, due
care was given to select the studies where both reference site (bare
land/fallow land) and grass treatments were present. The reference sites
were adjacent to the grass-treated field/plots within the same landscape and
similar slope. Therefore, we excluded studies where the reference site was
either missing or was away from the study site. The conservation use
efficiency (CUE) was calculated by the following formula (Khola and Sastry,
2005):
Relative performance of different grasses used as CGB was evaluated by using
a comprehensive performance index (CPI). The following formula was used to
compute CPI values of different grasses (Sudhishri et al., 2008):
Additionally, relative reversibility of erosion/water loss and relative yield gained due to adoption of CGBs were computed by using the following formulas:
Impact of grasses in arresting soil loss and runoff.
Values in the parentheses are mean
Relative reversibility of erosion/water loss:
India is home to about 1225 species of grasses, the majority of which grow
well in tropical and subtropical regions (Prakash et al., 1999). These grasses
can be used as live bunds in arresting soil erosion. The efficacy of CGBs in
increasing the opportunity time for infiltration and consequent profile
recharge has also been reported by other researchers (Sharma and Bhatt, 1996;
Prakash et al., 1999). In this meta-analysis, based on 25 observations, we
quantified the general potential of vegetative barriers to reduce runoff and
soil loss (Table 3). The overall result of the meta-analysis showed that
infiltration capacity increased approximately 2-fold after planting grasses
across the slopes in agricultural fields (95 % confidence level).
However, it is interesting to note that the mean runoff values were
statistically insignificant in the case of combined treatment of grasses along
with structural measures. This may be due to very high standard deviation
(SD) values obtained for vegetative barrier. These higher values indicate considerable heterogeneity in the observation which needs to be verified. Although
70 % of data showed similar variation, a few higher values were not
in expected lines, which might have caused this uncertainty. In the case of the Doon
Valley region, through comparing the impacts on soil wetting pattern, infiltration
rate and sorptivity, it was observed that
Runoff and soil loss values in CGB plots were lower than the control plots.
The data show that runoff varies between 11.26 and 62.60 % with a mean
value of 37.71 % and soil loss varies between 0.53 and
30.90 Mg ha
In terms of soil loss, the vegetative barrier of
Site-specific suitable grasses for contour grass barrier (CGBs).
A study by Shrimali (2000) revealed that
Relative merits of contour grass barrier (CGBs) in different land slopes.
Relative reversibility of erosion/water loss –
The conservation efficiency of CGB varies with grass types and site
conditions in different regions. However,
Analysis of variance through
Comparative comprehensive performance index of vegetative barrier.
Values in the parentheses are weights assigned to the respective attributes.
Variations in soil erosion amounts paralleled to some extent those of runoff in all the slope classes except in the lower slope range (Table 5). The protective actions of various CGBs are very clearly shown by the soil loss values, which show that between 141 and 107 % reversibility in soil loss can be achieved through adoption of CGB. The relative reversibility of water loss provided by CGBs compared to control was 52.6 % for the < 2 % slope and 55.5 % for > 4 % slopes. Favorable soil conditions created by CGBs resulted in an increase of yield in all slope ranges. The significantly higher yield in CGB treated sites may be due to either better moisture regime or higher nutrients or may be depending on both the retention of runoff and deposition of fertile sediment by the CGBs. The relative yield gained by CGBs varied between 44 and 53 %, with highest value in the 2–4 % slope.
A clear picture about the relative merit of CGBs was determined through
development of a CPI for different grasses (Table 6). Hybrid Napier grass
(
Grasses, shrubs and tree barriers in combination with structural measures (bioengineering measures) are known to be beneficial for soil and water conservation and have many relative advantages over structural interventions. Reinforcement by live roots which bind soil particles and underground decomposed biomass provides stability to aggregated soil. Plant detritus on the soil surface acts as a cushion for dissipating kinetic energy of rain drops. This aboveground biomass upon its subsequent decomposition also adds to the soil humus and increases infiltration, soil water-holding capacity and stability of aggregates (Prakash et al., 1999).
The data from Table 3 show that the use of grasses led to a significant
decrease in runoff from 25.53 % in control to 9.37 % with structural
conservation measures. Soil loss also has a significant decrease from 1.88 Mg ha
Earthen bund and earthen bund with broom grass was found to be more effective
in soil moisture conservation at 4 and 8 % slope as compared to other
treatments (Fig. 1). The study conducted on
In India about 12.0 Mha of area is represented by permanent pasture and
grasslands, majority of which is confined to the tropical areas (Roy and
Singh, 2013). Since this pasture land and grasslands are severely affected
by soil erosion, special attention should be given to their management to
reverse the process of degradation. Our synthesis of the meta-analysis
revealed that, by managing the grassland with cut and carry system,
rotational grazing and control grazing can greatly reduce the water and soil
loss and helps in the reversing the land degradation process. Similar
phenomena have been reported by Misri (2003) and Pathak and Dagar (2015),
especially for the lower Himalayan and Shivalik grassland, where severe
biotic pressure is imposed by both sedentary and migratory grazers. The
grazing intensity in the country is as high as 12.6 adult cattle units per
hectare (ACU ha
Complimentary role of grasses in enhancing soil profile moisture at 4 and 8 % slope.
The data (Table 3) show that runoff varies between 11.30 and 33.40 % with
a mean value of 24.33 % and soil loss varies between 1.52 and
3.28 Mg ha
Impact of grasses in reducing runoff in lateritic soil.
Impact of grasses in reducing soil loss in lateritic soil.
Impact of grasses in reducing carbon loss in lateritic soil.
In the hilly region of northeastern Himalayas, the alternative land use
systems help in reducing soil erosion systems and SOC loss to a substantial
extent. Higher root biomass of the grasses, particularly
Human-induced changes due to land use intensification and overgrazing have caused some severe and extreme states of land degradation that may prove to be more difficult to restore under the ongoing practices. The present meta-analysis clearly revealed that suitable conservation measures, especially the vegetative and biological practices, greatly assist in reversing the land degradation process for both cropland and grasslands.
Most soil erosion control measures implemented in cultivated fields are
physical structures. However, these physical structures were reported to be
less acceptable due to the high cost of their construction and maintenance. The
meta-analysis clearly showed that grass barriers potentially reduce runoff
and soil loss by up to 86.8 and 97.32 %, respectively. The relative
yield gained of various crops through CGBs at different slopes varied
between 44 and 53 %. However, the effectiveness of grass barrier, as
reported by several studies, is site-specific and depends mostly on slope
gradient, runoff volume and flow rate, size and density of sediment
particles, grass species, density, interval and width of grass strips,
underlying soil properties, and rainfall intensity and duration. According
to farmers' criteria based on CPI, the study revealed that
The present analysis also indicated that grass must be used as a vegetative strip to maintain soil quality in sloppy arable areas (8.5 Mha) of Indian hilly regions. Special emphasis on establishing grasses should be given to about 3 Mha of degraded pasture lands and 3.5 Mha of shifting cultivation areas in India to reverse the land degradation. Overall, we conclude that the use of grass barriers alone or in combination with structural measures and grassland management was effective and efficient for decreasing soil and water loss on sloppy croplands in tropical and subtropical regions of India. Thus, these practices should be strongly recommended and used widely in similar climatic regions. Similarly, the reduction in grazing intensity needs to be advocated for about 12 Mha of permanent pasture lands.
The authors declare that they have no conflict of interest.
We owe our sincere thanks to the ICAR for funding this project (ICAR – National Fellow) and the director of the Indian Institute of Soil and Water Conservation (IISWC) for providing facilities and support. Edited by: M. van der Ploeg Reviewed by: two anonymous referees