RESUMO

Nitrogen (N) is the most limiting nutrient for coffee production in Colombia. An adequate supply is especially important during the vegetative period of growth, since any deficiency during this short period is known to have lasting effects on subsequent coffee bean production. Urea fertilizer is commonly applied on the soil surface since steep slopes hamper incorporation into soil, a practice which increases the risk of N volatilization. Little information is available on N recovery during early growth stages under different fertilizer application practices. The aim of this study was therefore to provide a comparison of 15N uptake during the early vegetative growth stage under surface-applied and incorporation practices at two contrasting locations. The highest proportion of plant N derived from fertilizer (Ndff) occurred 60 days following application at the site with greater precipitation and soil organic matter, where surface application also increased the Ndff in roots and stems after 120 days. Although fertilizer N supplied approximately 20-29% of total plant N after 4 months, this fertilizer-derived N corresponded on average to only 5% of the total application, indicating that very little fertilizer (relative to how much is applied) reaches plants during this time. Apart from the difference in Ndff observed at the wetter site, there was no effect of application method on dry weight and macronutrient content in different plant components, root to shoot ratio, and leaf 13C content. However, site effects were registered for most of these measurements, with the exception of total nutrient uptake. Similarly to Ndff trends, lower root/shoot ratio and higher concentrations of N, K, and Mg in aboveground biomass were found in the site with higher rainfall and soil organic matter, likely resulting from higher soil water and N availability. These findings provide new information useful as a direction for further research looking toward increasing NUE during the vegetative stage in Colombian coffee crops.

RESUMO

Recent observations across a 14-year restoration chronosequence have shown an unexpected accumulation of soil organic carbon in strip-mined areas of central Brazil. This was attributed to the rapid plant colonization that followed the incorporation of biosolids into exposed regoliths, but the specific mechanisms involved in the stabilization of carbon inputs from the vegetation remained unclear. Using isotopic and elemental analyses, we tested the hypothesis that plant-derived carbon accumulation was triggered by the formation of iron-coordinated complexes, stabilized into physically protected (occluded) soil fractions. Confirming this hypothesis, we identified a fast formation of microaggregates shortly after the application of iron-rich biosolids, which was characterized by a strong association between pyrophosphate-extractable iron and plant-derived organic matter. The formation of microaggregates preceded the development of macroaggregates, which drastically increased soil carbon content (-140 Mg C/ha) a few years after restoration. Consistent with previous theoretical work, iron-coordinated organic complexes served as nuclei for aggregate formation, reflecting the synergistic effect of biological, chemical, and physical mechanisms of carbon stabilization in developing soils. Nevertheless, iron was not the only factor affecting soil carbon content. The highest carbon accumulation was observed during the period of highest plant diversity (> 30 species; years 3-6), declining significantly with the exclusion of native species by invasive grasses (years 9-14). Furthermore, the increasing dominance of invasive grasses was associated with a steady decline in the concentration of soil nitrogen and phosphorus per unit of accumulated carbon. These results demonstrate the importance of interdependent ecological and biogeochemical processes, and the role of soil-plant interactions in determining the success of restoration efforts. In contrast with previous but unsuccessful attempts to restore mined areas through nutrient application alone, iron-mediated stabilization of vegetation inputs favored the regeneration of a barren stable state that had persisted for over five decades since disturbance. The effectiveness of coupled organic matter and iron "fertilization," combined with management of invasive species, has the possibility to enhance terrestrial carbon sequestration and accelerate the restoration of degraded lands, while addressing important challenges associated with urban waste disposal.

Assuntos

RESUMO

Opencast mining causes severe impacts on natural environments, often resulting in permanent damage to soils and vegetation. In the present study we use a 14-year restoration chronosequence to investigate how resource input and spontaneous plant colonization promote the revegetation and reconstruction of mined soils in central Brazil. Using a multi-proxy approach, combining vegetation surveys with the analysis of plant and soil isotopic abundances (delta13C and delta15N) and chemical and physical fractionation of organic matter in soil profiles, we show that: (1) after several decades without vegetation cover, the input of nutrient-rich biosolids into exposed regoliths prompted the establishment of a diverse plant community (> 30 species); (2) the synergistic effect of resource input and plant colonization yielded unprecedented increases in soil carbon, accumulating as chemically stable compounds in occluded physical fractions and reaching much higher levels than observed in undisturbed ecosystems; and (3) invasive grasses progressively excluded native species, limiting nutrient availability, but contributing more than 65% of the total accumulated soil organic carbon. These results show that soil-plant feedbacks regulate the amount of available resources, determining successional trajectories and alternative stable equilibria in degraded areas undergoing restoration. External inputs promote plant colonization, soil formation, and carbon sequestration, at the cost of excluding native species. The introduction of native woody species would suppress invasive grasses and increase nutrient availability, bringing the system closer to its original state. However, it is difficult to predict whether soil carbon levels could be maintained without the exotic grass cover. We discuss theoretical and practical implications of these findings, describing how the combination of resource manipulation and management of invasive species could be used to optimize restoration strategies, counteracting soil degradation while maintaining species diversity.

Assuntos

RESUMO

Se evaluó la absorción foliar de nitrógeno inorgánico por depósito húmedo simulado en Abies religiosa, en un experimento factorial en vivero con plantas de tres años de edad y diseño al azar. Ocho tratamientos combinaron dos formas de N (NO3- y NH4 +), dosis aplicada (alta o baja) y escurrimiento del sustrato (con y sin). Se utilizó 15N para medir absorción de N. Las plantas se cosecharon y follaje nuevo, follaje con >1 año, ramillas, tallo principal y raíz fueron separados. La forma y dosis de N tuvieron efecto (p=0,001) en todos los componentes de las plantas, mostrando mayor absorción de N con NO3 - en dosis alta. Con escurrimiento hubo mayor absorción en ramillas, tallo y raíz. La mayor absorción de N fue en follaje nuevo. La absorción de NO3 - fue nueve y tres veces mayor que la de NH4 + en follaje >1 año y raíz, respectivamente. La absorción de N en tallo fue afectada por todos los factores e interacciones, pero en follaje >1 año solo afectó la forma y dosis de N. La cantidad promedio de 15N recuperada fue 1,8mg N por planta. La recuperación de N-NO3 - en la parte aérea alcanzó 57% pues las plantas no estuvieron expuestas a lluvia que lavase el N-NO3 - retenido en follaje y ramas. Los resultados sugieren un alto potencial de absorción por el follaje de A. religiosa. Los síntomas iniciales de saturación de N documentados en el Valle de México podrían acentuarse con aumentos de contaminación atmosférica. Los efectos del depósito de N pueden ocurrir por vía foliar antes que el N del suelo sea afectado.

To evaluate foliar absorption of inorganic nitrogen by simulated wet deposition in Abies religiosa, a greenhouse factorial experiment was established with three-year old seedlings using a randomized design. Eight treatments resulted from the combination of N form (NO3 - or NH4 +), dose (high or low) and runoff to the substrate (with and without). 15N was used to evaluate N absorption. Plants were harvested and current year foliage, foliage >1 year old, twigs, stem and roots were separated. The form of N and dose had significant effects (p=0.001) in all plant components, showing higher recovery with NO3 - as source and at a high dose. Runoff increased recovery in twigs, stem and roots. Current-year foliage showed the highest N absorption. N absorption by the whole plant was nine and three times higher with NO3 - than with NH4 + for foliage >1 year and roots, respectively. N absorption was higher in the stem than in other plant components, whereas foliage >1 year was the least sensitive component to N absorption. The average amount of N recovered per plant was 1.8mg N. Aboveground N-NO3 - recovery reached 57%, probably because the plants were not exposed to rainfall that could wash NO3 - from the canopy. The results suggest a high potential of N absorption by the foliage and branches of A. religiosa. The initial symptoms of N saturation documented in the Valley of Mexico may worsen if air pollution remains uncontrolled. The effects of N deposition may occur via foliage before soil N is affected.

Avaliou-se a absorção foliar de nitrogênio inorgânico por depósito úmido simulado em Abies religiosa, em um experimento fatorial em viveiro com plantas de três anos de idade e desenho ao azar. Oito tratamentos combinaram duas formas de N (NO3 - e NH4 +), dose aplicada (alta ou baixa) e escorrimento do substrato (com e sem). Utilizou-se 15N para medir absorção de N. Realizada a colheita das plantas, a folhagem nova, folhagem com >1 ano, ramas, caule principal e raiz foram separadas. A forma e dose de N tiveram efeito (p=0,001) em todos os componentes das plantas, mostrando maior absorção de N com NO3 - em dose alta. Com escorrimento houve maior absorção em ramas, caule e raiz. A maior absorção de N foi em folhagem nova. A absorção de NO3 - foi nove e três vezes maior que a de NH4 + em folhagem >1 ano e raiz, respectivamente. A absorção de N no caule foi afetada por todos os fatores e interações, mas em folhagem >1 ano somente afetou a forma e dose de N. A quantidade média de 15N recuperada foi 1,8 mg N por planta. A recuperação de N-NO3 - na parte aérea alcançou 57% pois as plantas não estiveram expostas a chuva que lavasse o N-NO3 - retido na folhagem e ramas. Os resultados sugerem um alto potencial de absorção pela folhagem de A. religiosa. Os sintomas iniciais de saturação de N documentados no Vale do México poderiam acentuar-se com aumentos de contaminação atmosférica. Os efeitos do depósito de N podem ocorrer por via foliar antes que o N do solo seja afetado.