Soil Organic Matter and Grain Yield of Rainfed Maize in Luvisols of Campeche, México

The organic matter is an important soil component, due its favorable effects on soil physical and chemical properties and, by consequence, on crop yields. So, the objective of this work was quantify the changes through the time of soil organic matter in rainfed maize (Zea mays L.) system, in a period of 30 years of continuous crop and in relation to system of natural vegetation system, and also the effects of these changes on the crop yield, in Luvisols soils of State of Campeche, México. In production cycle of 2004, 53 farmer plots in rainfed maize system of different land use time and five sites of forest natural vegetation system were sampled for soil, to quantify physical and chemical properties of soil, and it was taken data about climate, system management and crop yield. The data were analiced by regression analysis, considered the organic matter as function of land use time and factors of soil and management of system, and the crop yield as function of soil organic matter and factors of soil, climate and management of system. The soil organic matter di-minished from 5.68% in forest natural vegetation system to 3.59% after 16 30 years of cultivation, and the soil annual incorporation of vegetative mulch (weeds and stubble of maize) increased the organic matter from 0.14% after 1 - 5 years to 0.46% after 16 - 30 years of cultivation. Without fertilization, the diminution of soil organic matter caused 907 kg∙ha −1 less of maize yield, and the fertilization with 200 kg∙ha −1 of diamonic phosphate (36 kg∙N∙ha −1 and 92 kg P 2 O 5 ha −1 ) increased the maize yield 1224 kg∙ha −1 after 1 - 5 years and 1421 kg∙ha −1 after 16 - 30 years of cultivation, but not compensed less maize yield of 711 kg∙ha −1 due the diminution of soil organic matter.


Introduction
Organic matter is an important component of the soil, as it has favorable effects on physical, chemical and biological properties, and consequently, on the productivity of agricultural systems [1] [2]. Among its effects is: its cementing action on the particles, which promotes the formation of aggregates, which increases the flow of water, air and heat, and the capacity of infiltration and retention of water; in the process of decomposition and mineralization of organic waste, it provides macronutrients, especially N, P and S, as well as micronutrients; increases the capacity of exchange of soil cations, thereby reducing nutrient losses from leaching; and, biological activity increases, with its beneficial effects on production systems [3] [4] [5] [6]. The organic matter content of the soil is a function of the relationship established between the addition of organic waste and the rate of mineralization over time, and depends on the temperature and humidity conditions of the medium, pH, nutrient content and type and amount of soil colloids, the composition of organic waste and the conditions of aeration to which the soil is subjected [6] [7] [8]. In the systems of annual crops, especially intensive and mechanized, a decrease in the organic matter of the soil tends to occur, due to the greater removal and aeration of the soil by the tillage, which favors the decomposition and mineralization of the organic matter by the flora soil microbial [9] [10] [11]. Another cause of the decrease in soil organic matter is the burning of crop residues, which decreases the incorporation of organic waste into the soil [4] [12]. The study on the changes of the organic matter of the soil in the systems of annual crops is done observing the evolution of them in the time, from the natural ecosystem, and the effect of these changes on the productivity of the system by, if necessary, the decrease in crop yields [6] [13] [14] [15]. In the state of Campeche, Luvisoles soils occupy about 270,000 ha [16]. These soils were covered by medium forest and high perennial rainforest [17], and as the agricultural frontier expanded since the 1960s, they have been gradually incorporated into agriculture and livestock [18]. In this type of soil, the rainfed corn system is cultivated in approximately 140,000 ha [19], while a large part of the irrigation units (24 thousand hectares) are located in these soils, sowing mostly with vegetable and fruit crops [20]. The favorable physical characteristics of the Luvisols, as well as climatic conditions with annual rainfall of 900 to 1200 mm and, where appropriate, the availability of irrigation, has resulted in them having been subjected to intensive exploitation, both of annual and fruit crops like grasslands, characterized by high mechanization, burning of crop residues and low fertilization for the former, and poor management of the prairies. This has caused the deterioration of the soil, by loss of organic matter and fertility and by compaction, and finally a decrease in productivity [21] [22]. In non-disturbed ecosystems in Luvisoles of the State of Yucatán, Pool y Hernández [23] found soil organic matter contents of 26.2% for a depth of 0 to 3 cm, which decreased to 7.5% after four years of cultivation with a rainfed corn system. In these same ecosystems, Mariaca et al. [24] observed decreases in soil  [25] and Cuanalo et al. [26] found organic matter values of 2.8% to 3.0% in the first 20 cm, after 24 to 28 years with corn monoculture. A common practice in the Luvisols of the state of Campeche is the burning of crop and weed residues, which decreases the incorporation of organic matter into the soil, and may be the cause of its decrease, although Pool y Hernández [23] and Mariaca et al. [24] found no significant effect of burning crop residues on the organic matter content of the soil in the first 10 cm in a period of six years under corn cultivation. Regarding the effect of the decrease in soil organic matter on crop yields, Mariaca et al. [24] found a sharp decrease in yield of rainfed corn system after six years of continuous use with this crop, which was recovered with the application of fertilizers (values not indicated). On the other hand, Mariaca et al. [24] found increases in yields in rainfed corn system after eight continuous cycles in Luvisoles and Litosoles soils, of: The objective of the present study was to determine the changes in the organic matter content of the soil in the rainfed maize system, over a period of 30 years and with respect to a natural forest vegetation system, and the effect of these changes on the corn grain production, in Luvisoles soils of the State of Campeche.
The climate is AW1, which corresponds to a warm sub-humid with summer rains, with annual rainfall of 900 to 1200 mm, of which 85% occur between May and October, and average annual temperature of 25.5˚C to 26.4˚C [16] [26]. In the rainfed corn system, planting is done when the rains begin, which occurs in

Physical Properties of Soil
The soils of the different periods of time presented depths greater than 1.5 m and slopes less than 1%, and Table 1 Table 2 shows the contents of soil organic matter at a depth of 0 to 20 cm per period of time of land use for the rainfed maize system and the natural forest   The values of organic matter in the periods of 21 to 30 years are higher on average than those of the period of 16 to 20 years, in 0.29% and 0.31% respectively, in which other soil or management factors that favored the accumulation of organic matter in the soil, but not determined. Also, these same values are higher than those indicated by Turrent et al. [25] and Medina et al. [22] for the same soils and a continuous corn cultivation period of 28 years, from 2.8% to 3.0%.

Changes in Organic Matter over Time
On the other hand, although the color of the soil presented variations between periods of time, these were not related to the content of organic matter, as could be expected, since higher levels of organic matter are expected to have darker The regression model did not include the auxiliary variable for the SVN, which means that it explains the decrease in organic matter as a function of the time of land use and in relation to the SVN; On the other hand, it also did not include soil factors, such as the physical and chemical properties considered, that could have affected the behavior of the organic matter, from which it is deduced that its change in time was due to a greater extent to the time of use of the ground. The decrease in organic matter over time in soils under annual crops, and from the previous natural vegetation system, by the action of microorganisms, is widely documented in the literature [9] [10] [11]. For given soil and climate conditions, the decomposition and mineralization of organic matter will be greater the greater the removal of the soil by tillage and the non-incorporation or burning of crop residues. In the present case, although producers who have their own machinery follow a system of soil management determined over time, for producers who do not have machinery the situation tends to be different, since they do not always have the necessary machinery and on time. This makes its soil preparation system differ over time and, consequently, it is difficult to determine the effect of soil removal and waste management on soil organic matter.
The soil preparation work carried out by the producers are: a plow pass, by a few; from none to two steps of semi-heavy dredge; and, from none to three steps of light dredge. On the other hand, producers may or may not perform a mechanical cut of weeds that develop after harvesting or burning of them and crop residues, before the start of soil preparation for the next crop cycle. From all these works, it was found that the mechanical cutting of weeds and their incorporation into the soil, together with the crop residues, with the soil preparation work, increased the organic matter, according to the following regression model:  Table 3 shows the estimated average contents of soil organic matter over time in the SMT, without and with the incorporation of organic waste. In the period of 1 to 5 years, the highest organic matter content due to the incorporation of waste was 0.14%, a value that increased to 0.46% in the periods of 16 to 30 years.
Although the incorporation of waste increased the organic matter content of the soil, which indicates the importance of this practice, this increase did not compensate the losses of organic matter due to the continuous use of the soil, with temporary corn in the present case.

Effect of Soil Organic Matter on Production
The According to the production function, Table 4 shows the estimated average

Conclusions
In Luvisoles of the State of Campeche:  The organic matter of the soil decreased in the temporary maize system, from the natural forest system of previous forest up to 16