Use of cover crops in the southern Amazon region: What is the impact on soil physical quality?

Abstract

Introducing cover crops during the rotation system and off-season can provide numerous benefits to the agricultural system, such as increased carbon stock and improved soil aeration. However, in the southern Amazon region, owing to the hot and humid climate, the cover crops used in agricultural production systems may not have the same beneficial effects in the Cerrado region. This makes it necessary to introduce and study new species of cover crops that have the potential to improve soil quality. The objective of this study was to compare, over time, the dry matter production of eight cover crops, and the effects of their cultivation on the physical attributes of the soil in the southern Amazon region. The experiment was conducted in a randomized complete block design with analyzes carried out over the 3 years of the experiment, in three replicates. The treatments were composed of eight cover crops: crotalaria (Crotalaria spectabilis), pigeon pea (Cajanus cajan), velvet-bean (Mucuna aterrima), buckwheat (Fagopyrum esculentum), ruziziensis grass (Urochloa ruziziensis), brizantha grass (Urochloa brizantha), pearl millet (Pennisetum glaucum), and finger millet (Eleusine coracana), and three evaluation years (2014, 2015, and 2016). The cover crops used in the experiment were selected based on their adaptation to the growing conditions and because they have characteristics of agricultural interest. In addition, cover crops chosen can be used in soybean-maize rotation systems, soybean-maize succession systems, as well as in agriculture-livestock integration systems through integrated cultivation systems. Using undisturbed soil samples, were analyzed the physical variables: porosity, mechanical resistance to root penetration and water retention. Pore size distribution was calculated using the first derivative of the equation of van Genuchten and S index was determined by the angle of inclination of the soil water retention curve inflection point. The results show that leguminous and grassy cover crops act differently under physical soil attributes over time. The results indicated that the cultivation of grasses, such as P. glaucum, increased the values of macroporosity (0.18 m3 m−3) in the 0.00–0.10 m layer at the end of the third year of cultivation, consequently, reduced the value of soil density (up to 1.14 Mg m−3) and mechanical resistance to root penetration (up to 1.77 MPa). The largest accumulation of dry matter was obtained with the cultivation of U. brizantha, which consequently proved to be the best cultivation option in the off-season in the southern Amazon region. The study provides a better understanding of the influence of cover crops on the physical attributes of the soil, which will contribute to making the correct recommendation of cover crops that can improve the physical properties of the soil in agricultural frontier regions in the southern Amazon rainforest.