Francisco Matus

Effects of Soil Texture, Carbon Input Rates, and Litter Quality on Free Organic Matter and Nitrogen Mineralization in Chilean Rain Forest and Agricultural Soils

Abstract Nitrogen (N) mineralization in soil depends on carbon (C) input of plant materials, site environmental conditions, and soil texture. Empirical correlates of N mineralization can be difficult to interpret, however, because of interactions among these factors. A multiple regression model relating N mineralization to C input rates, litter quality, and soil texture in Chilean temperate rain forest soils was developed. Nitrogen mineralization rates ranged from 3.0 to 5.7 mg kg−1 soil d−1 depending on C input rate and soil texture but were not influenced by litter quality. We compared C storage of both forest and long‐term‐cropped soils with the protective capacity (i.e., the expected maximum C pool associated with the clay and silt fractions). Soil organic C associated with the fine fraction of forest soils was significantly greater than the calculated protective capacity, with clay‐rich soils averaging 141% more C than this limit and coarser soils having 56% more than predicted. In contrast, C content of cropped soils was well below the calculated protective capacity, averaging ∼32–60% less than this limit, showing the potential of these soils for sequestering C. The results were consistent with the finding that N mineralization was positively correlated with the amount of free organic matter associated with the sand‐size fraction in forest soils. The study illustrates that (i) the capacity of soils to preserve soil organic C in clay‐ and silt‐sized particles was greater than that of agricultural soils and (ii) in highly saturated soils, the N mineralization is a function of the quantity of organic‐matter input, which in turn accumulates as free organic C in the sand‐size fraction.

A simple model for estimating the contribution of nitrogen mineralization to the nitrogen supply of crops from a stabilized pool of soil organic matter and recent organic input

A simple model was developed to estimate the contribution of nitrogen (N) mineralization to the N supply of crops. In this model the soil organic matter is divided into active and passive pools. Annual soil mineralization of N is derived from the active pool. The active pool comprises stabilized and labile soil organic N. The stabilized N is built up from accumulated inputs of fresh organic N during a crop rotation but the labile N is a fraction of total N added, which mineralizes faster than the stabilized N. The passive pool is considered to have no participation in the mineralization process. Mineralization rates of labile and stabilized soil organic N from different crop residues decomposing in soil were derived from the literature and were described by the first-order rate equation dN/dt =-K*N, where N is the mineralizable organic N from crop residues andK is a constant. The data were groupedK1 by short-term (0–1 year) andK2 by long-term (0–10 years) incubation. Because the range of variation inK2 was smaller than inK1 we felt justified in using an average value to derive N mineralization from the stabilized pool. The use of a constant rate ofK1 was avoided so net N mineralization during the first year after addition is derived directly from the labile N in the crop residues. The model was applied to four Chilean agro-ecosystems, using daily averages of soil temperature and moisture. The N losses by leaching were also calculated. The N mineralization varied between 30 and 130 kg N ha−1 yr−1 depending on organic N inputs. Nitrogen losses by leaching in a poorly structured soil were estimated to be about 10% of total N mineralized. The model could explain the large differences in N- mineralization as measured by the potential N mineralization at the four sites studied. However, when grassland was present in the crop rotation, the model underestimated the results obtained from potential mineralization.

SEASONAL VARIATION IN LEAF LITTER NUTRIENT CONCENTRATIONS OF VALDIVIAN RAINFOREST TREES

La reabsorcion de nutrientes desde tejidos senescentes es un aspecto importante en la economia de nutrientes de las plantas perennes, y una influencia relevante sobre la dinamica de nutrientes en los ecosistemas. Han sido poco exitosos los intentos de interpretar el significado adaptativo de la variacion en la reabsorcion de nutrientes, llevando algunos cientificos a enfatizar la necesidad de investigar las bases bioquimicas y fisiologicas de la reabsorcion. Una hipotesis sugiere que la reabsorcion esta controlada por el efecto sumidero dentro de la planta. De acuerdo a esta hipotesis, se esperaria que la proficiencia de reabsorcion tenga una correlacion positiva con la tasa de crecimiento. Dicho patron se esperaria no solo para comparaciones entre especies o poblaciones, sino tambien cuando se comparan individuos conespecificos creciendo a distintas tasas, o en estudios de la misma planta durante diferentes fases del ciclo anual de crecimiento. Medimos la variacion estacional de la concentracion de N y P en la hojarasca en cuatro especies arboreas del bosque valdiviano, poniendo a prueba la hipotesis del efecto sumidero. En las cuatro especies la concentracion de N y P muestra una marcada variacion estacional. Todas las especies mostraron una baja proficiencia de reabsorcion (alta [N] en hojarasca) en invierno, lo que esta de acuerdo con la hipotesis sumidero. Sin embargo, la mayoria de las especie presentaron una baja concentracion de N y P (alta reabsorcion) en otono. Dado que el declive otonal del crecimiento vegetativo deberia determinar un debil efecto sumidero, este resultado parece contrario a la hipotesis inicial

Descomposición de hojarasca de Pinus radiata y tres especies arbóreas nativas

Recent decades have seen widespread conversion of native forests, shrublands and farmland in south-central Chile to exotic tree plantations. However, little is known about the effects of these wholesale landuse changes on ecosystem properties and processes, with the notable exception of studies of site water balance. In this brief communication, we present the results of a comparative study of decomposition of leaf litter of Pinus radiata and three common native tree species, beneath exotic and native woody vegetation in south-central Chile. We aimed to assess the nutrient cycling implications of substitution or invasion of native vegetation by P. radiata. Litter samples of the four species were incubated in both environments, registering the percentage of dry weight loss after two and six months. Decomposition rates of all species were much faster during the first two months of incubation than during the four subsequent months. At both dates there were significant differences between species and between sites, with faster decomposition of all species beneath P. radiata. There was no evidence of interaction between species and site. After six months, species rank order for the percentage of weight loss was Nothofagus obliqua > P. radiata > Peumus boldus > Cryptocarya alba. Interspecific variation in decomposition rates was more closely correlated with specific leaf area than with litter nitrogen content. Given that litter of P. radiata decomposed slower than that of the deciduous N. obliqua, but faster than the sclerophyll evergreens, the consequences of substitution or invasion for decomposition processes are likely to depend on the composition of the native vegetation in question.

RELACIÓN ENTRE LA MATERIA ORGÁNICA DEL SUELO, TEXTURA DEL SUELO Y TASAS DE MINERALIZACIÓN DE CARBONO Y NITRÓGENO

CCCada suelo posee una limitada capacidad de proteger fisico - quimicamente la materia organica contra la biodegradacion en sus particulas de arcilla y limo. Las tasas de mineralizacion de carbono (C) y nitrogeno (N) de los suelos estarian relacionadas al grado de saturacion de C en estas particulas. En el presente estudio se investigo la hipotesis de que la disminucion del C en la arcilla y limo a medida que el contenido de estas fracciones aumenta en los suelos se debe a una disminucion en la superficie especifica en las particulas de arcilla y limo. Tambien se investigo la relacion entre las tasas de mineralizacion de C y N con el grado de saturacion con C en las particulas de arcilla y limo. Seis suelos cultivados (cereales) y tres suelos de praderas (Medicago sativa L.) fueron muestreados a 0-20 cm de profundidad. Todos los suelos fueron dispersados con ultrasonido y gravimetricamente separados en fracciones > 50 µm (arena) y < 50 µm (arcilla y limo). A todas las muestras y al suelo entero se les determino el C - organico y la superficie especifica con etilenglicol. Por otra parte, se probo que existe una estrecha relacion entre el grado de saturacion con C en las particulas de arcilla y limo con las tasas de mineralizacion. Sin embargo, tambien se encontro una buena relacion entre el C-organico total del suelo y las tasas de mineralizacion. Esto ultimo coincidio con el hecho de haber encontrado una buena correlacion entre el C-organico del suelo y el grado de saturacion. Nuestros resultados explican parcialmente la hipotesis de que las tasas de mineralizacion estan mas relacionadas al grado de saturacion que a la textura (limo + arcilla) y al contenido de materia organica del suelo.

Organic matter stabilization in two Andisols of contrasting age under temperate rain forest

Recent studies with Andisols show that the carbon (C) stabilization capacity evolves with soil age relative to the evolution of the mineral phase. However, it is not clear how soil mineralogical changes during pedogenesis are related to the composition of soil organic matter (SOM) and 14C activity as an indicator for the mean residence time of soil organic matter (SOM). In the present study, we analyzed the contribution of allophane and metal–SOM complexes to soil C stabilization. Soil organic matter was analyzed with solid-state 13C nuclear magnetic resonance spectroscopy. Additionally, the soil was extracted with Na-pyrophosphate (Alp, Fep) and oxalate (Alo, Sio, and Feo). Results supported the hypothesis that allophane plays a key role for SOM stabilization in deep and oldest soil, while SOM stabilization by metal (Al and Fe) complexation is more important in the surface horizons and in younger soils. The metal/Cp ratio (Cp extracted in Na-pyrophosphate), soil pH, and radiocarbon age seemed to be important indicators for formation of SOM–metal complexes or allophane in top- and subsoils of Andisols. Changes in main mineral stabilization agents with soil age do not influence SOM composition. We suggest that the combination of several chemical parameters (Alp, Fep and Cp, metal/Cp ratio, and pH) which change through soil age controls SOM stabilization.

Soil carbon controlled by plant, microorganism and mineralogy interactions

Rhizosphere, a thin area of soil surrounding roots receiving carbon (C) exudation from plants, represents a site of intense competition for available C and nutrient between surface-reactive particles and soil microorganisms. This competition can reduce the amount of available C to a critical level, it becomes limiting for microbial growth and soil organic matter decomposition. On the other hand, acceleration or retardation of decomposition of soil organic C caused by root activity is termed rhizosphere priming effect (RPE). This effect has been increasingly recognized to play a crucial role on native C destabilization as is influenced by fresh C availability, microbial activity and soil mineralogy such as crystallinity of clay minerals and Al-, Fe-oxides. Combining microbial ecology and soil mineral interactions, we can understand how soil characteristics and climate change can influence below ground competition and finally RPE. In this review, we focus on the competition for available C in soil, limiting our analyses to the interaction at rhizospheric space, where most processes between microorganisms and mineral phase occurs.

The decomposition of wheat and clover residues in soil: measurements and modelling

A computer model is described that is able to trace the fate of nitrogen in crop residues added to field soils. Clover and wheat residues labelled with 15N were added to a clay and a sand soil and the fate of the label traced over a period of almost 16 months under field conditions. Using a simple function to retard the decomposition of crop residues according to how much fibrous tissue they contain, the model was able to estimate the organic N remaining in soil, and the mineral N and microbial biomass N derived from the crop residues. It proved necessary, however, to postulate the existence of a pool of organic matter derived from crop residues that was more labile than native humus in soil.

Is the Walkley-Black Method Suitable for Organic Carbon Determination in Chilean Volcanic Soils?

Although dry combustion (DC) carbon dioxide (CO2) is an accurate method to measure total soil carbon (C), it is a rather expensive one. Therefore, wet oxidation by the Walkley–Black (WB) method is widely used in acidic Chilean volcanic soils, although there are no studies comparing both WB and DC. The aim of this article was to compare DC and WB in a range of volcanic soils containing between 2% and 9% of soil C on a regional scale. Results indicated that the recovery (R) of soil C by WB with respect to DC varied between 70% and 82%. Consequently, the correction factor (100 / R) ranged between 1.26 and 1.47. The standardized major axis regression analysis indicated that the slope and the intercept of the fitted line on volcanic soils were similar to 1:1 line. In conclusion, WB was an economically suitable method to determine the soil C content of Chilean volcanic soils.

GRAZING MANAGEMENT, AMMONIA AND NITROUS OXIDE EMISSIONS: A GENERAL VIEW

The grazing management of grassland has a direct effect on nitrogen (N) recycling. This is an important reason why management has become an alternative to improve the grassland production and quality, in turn to make it more suitable for the environment. However, the livestock system intensification induces changes in the natural dynamics of the N cycle, accelerating gas emmisions (e.g. ammonia, NH3 and nitrous oxide, N2O) and leaching losses from soil under grazing. When the amount of N in the environment increases, there is an impact on smog episodes, global warming, stratospheric ozone depletion, acid rain and eutrophication of fresh water. There are different techniques to evaluate the gases emitted from the soil. This klonowledge is useful to design the strategies to reduce the negative consequences of theses gases on the environment. In this review, the effect of grazing managements on N gas emissions from soils and the current techniques for N gas emission measurements in the field and laboratories conditions are discussed.