Geology, Land Use Systems and Soil Fertility in the Yucatan Peninsula

The Yucatan Peninsula is constituted by an extensive and deep limestone and dolomite platform that emerged to the surface gradually in a northerly direction from the sea-bottom 25 to 6 million years ago in the Tertiary and Quaternary Period (Duch 1991). The peninsula was formed from whatever shallow-water coastal drift materials happened to lie on the hard crust immediately prior to uplift (Duch 1991).

Limestone as parent material and the limited weathering explain the shallowness and stoniness of the Yucatan soils. The substratum is of calcareous origin, of which the carbonates represent 95% (Pool 1986), and it is composed of calcium (CaCO3) and calcium-magnesium carbonate (CaMg(CO3)2, dolomite) (Wilson 1980). The permeability of the soils in Yucatan is high and this characteristic does not permit the formation of superficial water, hampering the formation of rivers. The filtration of water led to a karstified subterranean network of water bodies (Wilson 1980). These caverns occasionally breach to the surface and create natural wells that are known as “Cenotes”. These are holes of different sizes, which are a result of the dissolution of CaCO3 (Hernandez X., 1959).

The majority of the state presents an undulated and low relief. Only in the south some major elevations can be found due to a folding of the limestone platform in the Tertiary. The solid limestone is locally called Chaltun with a calcareous, friable, and whitish layer underneath, known as Sahkab, which is able to store infiltrated water. Limestone on the soil surface is easily cracked and penetrated by plant roots, which explains the abundance of stones (Duch 1994).

The Yucatan soils present a mosaic of different characteristics: there is a great morphological variation, factors as colour, localization, depth, fertility, water, content of stone (Duch 1991, 1994). The differences in colour are associated principally to the content of organic matter, which give the basic coloration of black, red-brown and red, with high (black soils) or low

I. General Introduction

Since pre-Columbian times, different agricultural and forestry practices have been developed, modified, and carried out by the Mayan population in the peninsula. At present, the traditional shifting cultivation (milpa) in co-existence with secondary forestry, and homegardens (solares), are the land use systems where the families can self-satisfy their basic necessities, with essential products for their diet as for example maize, which is the principal nutritional source, fruits, and vegetables.

Currently, the milpa system is developed in one third of the soil in Yucatan (Moya et al.

2003). The most important crop of the milpa is maize (Zea mays) followed by squash (Cucurbita pepo and C. moschata) and beans (Phaseolus vulgaris and P.lunatus). In parts of the milpa small patches are used for horticultural species like chillies (Capsicum annum), tomatoes (Lycopersicum esculentum), watermelon (Citrullus lanatus), jícama (Pachyrrhizus erosus), manioc (Manihot eculenta), sweet potatoes (Ipomea batatas), xcucut makal (Xanthosoma yucatanense) and cucumber (Cucumis sativus) (Hernandez X. et al. 1994).

The milpa has different cycles; the first task for the farmer is to select a suitable area and to evaluate the soil, relief, and existing vegetation. The farm worker clears a piece of forest sized between one and two hectares using the slash-and-burn method. The burning takes place at the end of the dry season during the month of March or April, when the slashed vegetation has dried and the beginning of the rain is forthcoming. Normally the milpa system has a short period of cultivation, around two years with long periods of fallow (15-25 years) but now with the increase in the population, changes in the land tenure and limited allocation; the fallow periods have been reduced (20-7 years) and the soils cannot recover its fertility between milpa cycles (Teran and Rasmussen 1992, Benjamin 2000, Weissbach et al. 2002).

Homegardens in Yucatan have been described as other important agricultural system for the Yucatecan families (Benjamin 2000). This system is composed of intimate, multi-story combinations of various trees and crops, in association with domestic animals and around homesteads (Anderson 1993). Despite the fact that the solares are less than one hectare in size, they have a great diversity of species and present three to four vertical floristic strata (Benjamin 2000). The principal function of the homegarden is food production; also to

I. General Introduction generate secondary products (medicinal plants, seasonings, utensils, firewood, etc).

Dominance of species in a vegetation analysis of solares in Yucatan was found by Xuluc (1995), among them: Annona squamosa, Apoplanesia panniculata, Brosimum alicastrum, Cedrela odorata, Citrus auratium, Cordia dodecandra, Ehretia tinifolia, Manilkara zapota, Melicoccus bijugatus, Musa paradisiacal, Spondia purpurea, and Talisia olivaeformis. Rico-Gray et al. (1990; quoted in Benjamin 2000) observed that due to modernization and developing processes, there is a tendency to changes in the structure and functions of the solares.

Several studies have been focussed on the fertility and quality in agricultural soils, and have shown a decline in the soil fertility of the peninsula (Perez et al. 1981, Zech et al. 1991, Weissbach et al. 2002). Zech et al. (1991) observed that some nutrient (P, N, Mn, and Zn) might be deficient. However, Weisbach et al. (2002) who studied the soil fertility in milpa systems with several fallow periods concluded that the nutrient status of the Yucatan soils is higher in comparison to other semiarid tropical soils, and have a rapid capacity of fertility regeneration but low productivity. In addition, Shang and Tiessen (2003) found a high organic matter content (50-150 g C. kg^-1) in some soil types (for example black soils) of the region.

As the peninsula receives low amount of precipitation this might be a reason for their low soil productivity. Generally in semi-arid regions, water, soil nutrients, and plant productivity, typically go through periods of high and low pulses (Schwinning and Sala 2004, James and Richards 2006, 2007). Short periods of high resource abundance are triggered by rainfall events, which despite the overall scarcity of rain, can saturate the resource demand of some biological processes for some time (Schwinning and Sala 2004). Rainfall input into a dry soil triggers a cascade of biogeochemical and biological transformations, which can vary in time from hours to years. For example, the liberation of nitrogen by microorganisms residing on the soil surface takes only hours (Cui and Caldwell 1997) and the demographic responses of primary decomposers and consumers that unfold can take years (Osfeld and Keesing 2000).

I. General Introduction It is known that the soil biological processes are water dependent, because the soil microbial activity is influenced by moisture conditions and, consequently the dynamic of nutrient transformation is affected. Both carbon (C) and nitrogen (N) mineralization rates increase for a few days following the rewetting of a dry soil (Fierer and Schimel 2002). Water may enhance C mineralization and stimulate microbial activity by acting as a solvent for organic substrates derived from litter. In addition, phosphorus (P) diffusion occurs in water-filled pore spaces in the soil, and their availability to the plants by the moisture is also affected (Misra and Tyler 1999). Higher moisture content allows these substances to diffuse through a greater proportion of the soil pore volume, making them more available to microorganisms and consequently high nutrients mineralization. However, in soils with high organic matter content, the drought, wetting and rewetting might be different and therefore have different effects on the mineralization processes and in nutrients availability.

Soil fertility is related to microbial characteristics (enzyme activities, microbial biomass, microbial turnover, and microbial population), soil physicochemical properties (pH, organic C content, and nutrient availability), vegetational response (plant biomass or yield and nutrient uptake), and climatological conditions (temperature and rainfall). Quantitative data on the chemical and physiological processes in the Yucatecan land uses are few. Most of the studies are focused on the structure, system inventory, species composition, diversity, and socio-economic aspects. Studies about litter decomposition and chemical characteristic have been reported, but information on the biological and biochemical dynamic and transformation of nutrients in the land uses in Yucatan are not well documented.