Recuperación de la calidad de Ultisoles mediterráneos degradados, mediante la aplicación de enmiendas y formas alternativas de uso

Abstract

RECUPERATION OF MEDITERRANEAN ULTISOLES'S QUALITY THROUGH THE APPLICATION OF INORGANIC AMENDMENTS AND ALTERNATIVE FORMS OF USE ABSTRACT Ultisols (Soil Survey Staff, 1999) ¡n the plateaux of the raña formations in central Extremadura teslífy to the presence of a former a subtropical climate with warm, wet summers; these conditions resulted in strong alteration and leaching, and in chemlcal properties including a very low pH that decreases with depth, hlghly deficient contents in exchange bases and P, and an exchange complex dominated by Al. In their current status, the soils are of little use for sustaining the typical crops of the Mediterranean climate under which the area is at present. The climactic vegetation of the area (vlz. holm oaks and replacement bushes) was largely removed over the 1940s to 1950s In order to cultivate the clear surfaces. This was a result of the shortage of food in the years following the Spanish Civil War and the availability of phosphorites in the nearby deposits of Logrosán (Cáceres). Tillage practices induced mineralization of organic matter in the surface horizon, and facilitated the release of exchange bases and nutrients that enabled cultivation of extensive surfaces with grain crops (ca. 85% of the cropped surface), particularly rye (Sécale cereale L.), forsome time (10 -25 years). After those years of tillage, the already low productivity of the soils dropped to inviable levels, so farmers eventually abandoned their fields. A fraction of (5 -15%) of the fíat surface of the plateaux was used to grow olive (Olea europaea L.). The trees, which were planted at distances of 9 m or even greater, are currently glving very low outputs (in the region of 5 -12 kg olives/tree), so the orchards are also being gradually abandoned. A minor fraction of the cleared surface was used to grow vine (Vitis vinlfera L.). The abandoned fields soon started to develop rather poor pasture with very sparse legume patches that were soon invaded by Cistaceae (Cislus ladaniferus L., Halimium occimoides (Lam) Wilk) and heather (Calluna vulgarls L., Erica umbellate L, E. arbórea L.). With time, a bush stratum took over that in some places was dominated by Cistus ladaniferus L., usually distant from evolved bushes richer in flowery species including madrone, heather and broom near cork oaks. Since the late 1990s the Department of Edaphology of the Polytechnic University of Madrid has worked towards improving the agronomic quality of these tilled soils ?many of which had previously been abandoned? by using lime and/or gypsum amendments. In this work, firstly, we studied the degrading effects of deforestation and cropping on the quality of the soils. To this end, we used as reference soils from relict surfaces under old cork oaks that were never deforested as reference. The quality - degradation binomial was studied on a selected series of surfaces covered to a variable extent with pasture and bushes which had been abandoned at different times, as well as a surface under olive trees which has been under continuous tillage ever since its natural vegetation was removed. This study revealed that the principal source of degradation of the soil parameters related to soil quality, both physical (bulk density, surface infiltration, proportion of water-stable aggregates), chemical (base and Al contents of the exchange complex) and biological (C02 release rate,?-glucosidase activity), was the dramatic reduction in organic matter (OM); so, the reference soil under cork oaks being classified in the Ultisol order, as belonging Humults sub-order (Soil Survey Staff, 1999) and all other soils, whether cropped or abandoned, as Xerults suborder. Some labile OM fractions (e.g. particulate organic matter (POM)) were more markedly affected by tillage than others. As a result of the foregoing, the degree of deterioration of soil quality was closely related to the length of the tillage period of the surfaces, which was longest for the soil under olive trees. Also, previously cropped soils which had been abandoned for years were found to exhibit very slow natural recovery by effect of their decreased Ca/AI ratio in the exchange complex relative to the reference soil; this ¡ncreased Al toxicity, which hindered development of natural vegetation and henee recovery of the original OM contents. Because degradation of the soils resulted from the decrease in OM contents by effect of their cultivation, we considered not only the application of lime and gypsum amendments to counler the action of Al, but also the use of forms not favouring the destruction of soil OM but rather, if at all possible, raising its contents. The measures considered included using the soils to grow pasture for cattle and game feeding purposes. We conducted a study about the effect of applying lime (sugar foam waste) and gypsum amendments (phosphogypsum) in order to (a) improve the properties of the exchange complex and alleviate or suppress the problems arising from excessive acidity and AI3+ contents, and deficient Ca contents, on biomass production by natural pasture (NP) and an improved pasture (IP) obtained by introducing new species boosting nutritional quality; and (b) correct the dynamics of OM in general and its more labile fractions in particular, as well as quality- and OM-related parameters of physical, chemical and biological nature. Soils were monitored similarly, treated or untreated with amendments, but managed with forage crops (FC), in this case, the soils were managed under reduced tillage including no mouldboard ploughing. The above-described studies were conducted on the platform of the Cañamero raña (Cáceres, Spain), where an experimental field was established in 2002 in a previously cropped and later abandoned area representative of a sizable fraction of the raña surface. The experimental design consisted of split plots with four replications per crop type and treatment where crops or uses included natural pasture (NP), improved pasture (IP) and a forage crop (FC). The amendments applied were sugar foam waste (SF), phosphogypsum + sugar foam waste (PG + SF) and a control (C). A total of 36 plots (4x3x3) were thus studied. After 54 months, the soils under the three types of use exhibited substantially improved biomass production by effect of the treatments. This was a result of the increase in pH and Ca/AI ratio. The sugar foam waste proved effective to suppress Al toxicity in the topmost 25 cm of soil; on the other hand, the phosphogypsum + foam mixture was effective down to the Bt1 horizon. The treatments raised total OM contents under all use types (particularly in IP amended with sugar foam waste). The effect was specially marked on particulate OM, the contents in which peaked in the topmost soil layer (0- 5 cm). All treatments (especially sugar foam waste by virtue of its favourable effects on root mass) raised particulate OM contents in the 5-25 cm layer. Finally, we studied the influence of the phosphogypsum rate used on the Ap horizon on biomass production by wheat (Triticum aestivum L.) of the Jabato variety. At low rates (up to 5 t/ha), the amendment increased biomass production; by contrast, rates in the region of 10 t/ha resulted in anomalous plant growth and rates above 30 t/ha reduced biomass production dramatically with respect to the controls. This dynamics was a result of fluorine contained in this by-product attacking silicates in the soil and releasing Al into the soil solution, thereby raising the activity of AI3+, AIOH3+ and AI(OH)2+, which are the most phytotoxic aluminum species.