Maize (Zea mays
L.) and beans (Phaseolus vulgaris
L.) are staple foods in Central America [1
] and are the two most important crops in the region, far ahead of coffee and cocoa, the main export crops. Over the last 53 years, the cultivation of maize in Central America has expanded in the region, and nearly nine million hectares of farmland are currently dedicated to this crop (increasing from 7.67 million hectares in 1961 to 8.93 million hectares in 2014, with an annual growth of 0.29%) [2
]. For the same period, the cultivated area of bean (dry and green) increased from 1.89 million to 2.47 million hectares (annual growth of 0.51%). Moreover, this increase was accompanied by increased productivity. The yield of maize cultivation rose at an average annual rate of 2.17%, from 974 to 3037 kg ha−1
between 1961 and 2014. For beans, the rate of increase was irregular, with the productivity of dry beans increasing by 1.0% per year, from 466 to 791 kg ha−1
, while that of green beans increased much faster, at 3.44% per year, from 1498 to 8992 kg ha−1
. The intensification of agriculture in Central America, and in Guatemala in particular, has been analysed by Klepek [3
], who highlighted the technological “Green Revolution”, which began in the 1950s with US development aid, in a Cold War context of geopolitical tension. Apart from the characteristics affecting specific regions of the planet, agricultural intensification is a global phenomenon [4
], and appears likely to continue in the coming decades, according to FAO forecasts, which suggest that by 2050 more than 80% of the world’s agricultural production growth will have resulted from the increased productivity of land currently under cultivation [5
The intensification of agricultural activity in Central America has profoundly changed maize and bean cultivation systems in the region. With respect to soil and biomass management, the traditional system, based on shifting cultivation patterns and on no-tillage farming, has given way to a new model characterised by permanent agriculture and by tillage using ploughs or hoes. Soil erosion by water is a major problem in tropical mountain areas [6
] and the above-described changes in the agricultural system have aggravated this problem [9
]. The new model has replaced woodland fallow with (mainly inorganic) fertilisation, which meets the immediate needs of crops, but does not compensate for the losses of organic matter from the soil due to mineralisation. Accordingly, there may be decreased stability of aggregates and greater vulnerability to soil erosion [10
]. Tillage also facilitates the mineralisation of organic matter, especially in tropical conditions [11
], and provokes similarly negative consequences.
From the economic and social standpoint, this soil degradation provokes the loss of the most fertile layers of soil, and thus reduces crop productivity [12
]. Agriculture is the means of livelihood for many families of small and medium-scale producers living in these mountainous areas, and its sustainability is threatened by the degradation of resources [15
]. Moreover, this circumstance may be aggravated in coming years by climate change. In recent years, temperatures have increased and there has been a higher intensity of precipitation–drought extreme events in the wet and dry regions, respectively, of Central America [16
], a tendency that is expected to continue [18
]. Furthermore, and especially relevant to the present study, climate models suggest that the isthmus will experience a greater frequency and intensity of extreme rainfall events in the form of hurricanes and tropical storms [17
]. This outcome is predicted to affect all the tropical zones of the planet, with rising sea surface temperatures being the main cause of this process [19
Over time, these changes may have severe environmental and economic consequences. In particular, extreme rainfall events can produce significant environmental impacts, greatly increasing the risk of erosion [17
], especially in mountain soils where vulnerability is most acute. In economic terms, a general loss of agricultural production is expected [17
], affecting the cultivation of maize in particular [23
In view of the uncertain environmental and economic context of Central America, measures must be taken to prevent erosion and the loss of soil fertility. With this aim in mind, studies have examined erodibility factors (both environmental and specifically with respect to agricultural management) to determine the erosion threshold [24
]. This concept, which has been defined as “the value (of the erodibility factor) beyond which the effective control of erosion is achieved” [26
], is of particular interest for practical purposes because it can be used to improve the planning of agricultural activity, from the viewpoint of soil conservation, which is in line with the goals of UNEP [27
]. Specifically for Latin America, knowledge of erosion thresholds could provide useful means of monitoring and responding to soil degradation, in actions such as those described by Lee et al. [22
], who sought to design strategies for local development in which climate change effects were taken into account. The need for such strategies had previously been highlighted by Eakin and Lemos [28
]. With respect to the methods to be employed in these strategies, the erosion threshold can be used to complement the Visual Soil Assessment method [30
], thus facilitating decision making aimed at sustainable land use.
Soil erosion by water depends, among other factors, on the types of crops involved and the different levels of vegetal cover they provide [31
]. The case of annual crops is particularly significant because they may experience significant differences in cover during the agricultural cycle, and therefore the impact of erosion may vary depending on the stage of development of this type of crop. Determining the erosion threshold in annual crops is especially complex because it requires taking into account different conditions of development and cover during the agricultural cycle.
The present study has the following aims, (1) to compare values for soil erosion by water at two stages of the crop development cycle (at 3 and 6 months after sowing) in the mixed cultivation of maize (Zea mays L.) and beans (Phaseolus vulgaris L.) and (2) to analyse the influence of the ground and crop canopy vegetal cover on this erosion and to determine soil erosion thresholds by means of the factors cited above.
The results obtained show that the threshold of 80% of ground cover by weeds and litter in cultures of maize–bean at 6 months after sowing did not correspond to effective erosion control. In the plots with the highest levels of ground cover, a large proportion of the surface area continued to be affected by soil erosion, at an average of 44.0% in plots with 80 to 100% cover. This figure is well above that observed in agricultural systems that achieve better erosion control results, such as agroforestry coffee systems, where only 10.5% of the soil area is reported to be affected [24
The forest canopy changes the kinetic energy of rainfall by altering the size and velocity of the raindrops, and thereby changes the potential for the initiation of splash erosion [36
]. The same effect could be expected in the case of crops, that is, the vegetal cover, comprised of the crop canopy cover (maize and bean) and of the ground cover (layer of weeds and litter), would protect the soil against the impact of rainfall and act as a barrier to surface runoff. The results obtained in this study of maize–bean plots at 6 months after sowing show that the soil cover provided by weeds was significantly correlated with the area affected by soil erosion, unlike the cover provided by crop canopy and plant litter. However, the variable that presented the best linear fit with the area affected by soil erosion was that of ground cover by weeds and litter. The litter layer did not correlate independently with the area affected by soil erosion, probably due to its low presence and considerable dispersal over the sampling plots. However, a clear relationship with erosion can be assumed, given that the dependent variable is best explained in terms of total soil cover. In other words, the results obtained highlight the importance of the litter layer in preventing erosion.
Previous studies of erosion in maize and bean cultivation have demonstrated the importance of plant litter ground cover in protecting against splash erosion and increasing resistance to surface water flow [25
]. In addition, plant litter is an important source of organic matter, increasing the stability of soil aggregates and enhancing resistance to erosion [39
Ground cover by weeds and litter in the maize–bean plots at 6 months after sowing contained a higher proportion of weeds (36.4 ± 4.4%) than of plant litter (13.3 ± 1.9%) (Table 1
). When focusing solely on cases of maize–bean cultivation with over 80% cover, the differences are even larger. In these cases, plant litter accounts for only 15.7% of total ground cover, compared to 75.5% covered by weeds (Table 8
). These values contrast with those reported in earlier studies of bean cultivation (16.7% and 35.4% weed and litter cover, respectively) [25
] and of maize cultivation (0–1.2% weeds and 0–44.2% litter) [37
The lower proportion of plant litter cover measured in the present study means that the soil was not effectively protected, as erosion was still present under the dense weed cover, while there was little plant litter. Under these conditions, the area affected by soil erosion remained high, because there was no surface litter cover to prevent runoff (Figure 8
(c2)). In short, the increased canopy cover did not compensate for the observed lack of a substantial litter layer.
The results obtained showed that plant cover at the 3-month stage was lower than at the 6-month stage because this is when the surface presence of weeds was reduced by soil inversion, using hoes. Under these circumstances, the ground cover was insufficient to exert much control over erosion. Moreover, the resulting disturbance of the soil further increased its susceptibility to erosion [25
]. In summary, the soil at this stage of the crop cycle was far removed from the ideal conditions for achieving erosion control, which accounts for the greater area affected by soil erosion.
The soil and vegetation management performed in the study area are key factors in explaining the considerable area of soil surface affected by erosion, and the impossibility of establishing an erosion threshold. Labrière et al. [8
] and Derpsch et al. [41
] argued that special attention should be paid to these factors in order to achieve effective control of erosion in tropical areas.
In the study area, soil loss was slight and widely dispersed during the two sampling periods. Soil loss was only recorded in 29 of the 43 study cases (14 at 3 months and 15 at 6 months). In the remaining plots, there was no erosion in rills and gullies during the sampling periods. Various reasons for this absence of erosion can be suggested. At the 3-month stage, there is a scant presence of rills because the soil is disturbed during tillage; this action eliminates the furrows and gullies that may have formed previously [31
]. At the 6-month stage, the large amount of weed cover limits the formation of rills [31