Mediadores do acúmulo e da estabilidade do carbono orgânico do solo no trópico úmido

Abstract

Sustainable management and the assessment of soil health indicators in the humid tropics are urgent and necessary. Historically, this region has faced food insecurity, rural poverty, and degradation of natural resources. Simple fertilization is not sufficient to improve soil health due to rapid nutrient losses and accelerated organic matter decomposition, making integrated strategies necessary for more efficient agricultural systems. In this context, this research aims to (i) estimate the effect of polyvalent cations on the accumulation and stability of soil organic carbon in tropical regions; (ii) determine key indicators of soil health in the humid tropics; and (iii) understand the dynamics of Ca and Si on ecophysiological indices, stabilized soil organic carbon (SOC) fractions, and nitrogen use efficiency in soils of the Amazonian periphery. In Chapter 1, a meta-analysis was conducted based on 14 studies published between 2003 and 2023, using random-effects models to evaluate the influence of the polyvalent cations Al, Fe, Mg, and Ca on SOC concentration. The results revealed distinct effects among the cations (p < 0.001), with Al being the most strongly associated with increases in SOC, followed by Mg and Ca. In contrast, Fe showed the smallest effect. Heterogeneity among studies (I² = 77.01%) indicated that factors such as soil class, pH, texture, and land use significantly modulate cation effects. Soils with pH between 4 and 6 and clay contents above 600 g kg⁻¹ favored SOC concentrations > 30 g kg⁻¹, particularly in the presence of Ca contents exceeding 40,000 mg kg⁻¹. For the subsequent chapters, a field experiment was conducted in the municipality of São Luís, Maranhão, Brazil, from October 2021 to July 2024. A randomized complete block design (RCBD) was adopted with four replicates and eight treatments: (1) urea; (2) legume; (3) legume + urea; (4) calcium + legume; (5) silicon + legume; (6) calcium + legume + urea; (7) silicon + legume + urea; and (8) calcium + silicon + legume + urea, under maize cultivation. In Chapter 2, the contribution of 17 edaphic variables to SOC and maize productivity was investigated using multivariate analyses (PCA, RDA, decision trees, and regression). Chemical variables—especially MAOC, Si, and, to a lesser extent, Ca—exerted the greatest effects on SOC and productivity. MAOC was highly significant (p < 0.001) and showed a strong correlation (R² = 0.87) with total SOC, whereas physical and biological variables, such as macroporosity and microbial respiration, showed secondary influence. In the final chapter, the response of the ecophysiological indices qCO₂ and qMIC, as well as SOC fractions (POC and MAOC) and nitrogen, to the studied treatments was evaluated. The results demonstrated that Ca and Si contributed to increases in the stable carbon fraction (MAOC) (p < 0.05), with Si standing out by promoting MAOC contents 32% higher than Ca when combined with legumes. Nitrogen concentration in maize plants was 72% higher under Si + legume than under Ca + legume (p < 0.05). Silicon also showed greater efficiency in the use of biological nitrogen, in contrast to Ca, which depended on synthetic N to achieve a similar effect. Overall, the results of this research reinforce that sustainable management strategies prioritizing the addition of stabilizing cations and Si, the promotion of MAOC, and the use of biological N sources can improve soil health, increase agricultural productivity, and contribute to carbon storage and stability in soils. These strategies are fundamental to mitigating climate change impacts and ensuring the long-term resilience of tropical agroecosystems.