Recalcitrant carbon and nitrogen in agriculture soils with residue accumulation and fertilization under tropical conditions

Abstract

Soil organic matter has a strong relation to total organic carbon, and about 85% of organic carbon consists of humic substances (HS), classified as humin (HU), humic (HA) and fulvic acids (FA), and denominated as recalcitrant carbon in soil. HS are formed by complex, heterogeneous and polydisperse molecules, which have significant influence on the soil physical and chemical characteristics. The study evaluates the hypothesis that agricultural soils treated with organic residues may present higher carbon stocks as presented in forest soils. The aim of this study was to evaluate alterations in recalcitrant carbon and nitrogen stocks in Oxisol (Forest-FL, unfertilized Brachiaria-UB, and fertilized Brachiaria-FB) and Cambisol (Forest-FC, Coastcross-CC, sugarcane-CA, and silage corn-SM) at surface (0.0-0.1) and subsurface (0.1-0.2 m), in the Zona da Mata in Minas Gerais state, Brazil. Result shows that fertilization, low soil disturbance and residual removal promoted increase of C and N content in HS, being close to native forests. Both, carbon and nitrogen recalcitrant, presented reduction with soil depth. HU ranged from 10.5 to 16.7 g kg-1 and presented the highest concentration compared to FA and HA. In Oxisol, FL and FB presented the highest SH concentration and demonstrate the positive effect of fertilization on carbon stocks improving soil quality in well-managed and productive areas. While, In Cambisol, FC and CC presented higher carbon and nitrogen in HS, mainly for HU and HA fractions, and SM showed the lowest concentrations in all fractions. Our results suggest that soil managements with lower soil disturbance and residual removal promotes increasing of carbon and nitrogen in recalcitrant fraction, with concentration close to native forests. Pasture should be fertilized to improve recalcitrant carbon and nitrogen stocks, avoiding process of degradation in tropical soil. It is an important outcome due to high levels of degraded areas in Brazil caused by inadequate use of soil mainly with pasture.

How does organic carbon operate in the pore distribution of fine-textured soils?

“…On the soil surface, the higher carbon content was associated with sugarcane while there was no difference between sugarcane and coffee in the subsurface soil (Figure 3A and B). Commonly, the highest contents of total organic carbon in the soil are found on the soil surface with a consecutive decrease in the subsequent soil layers (Almeida et al, 2019). Carbon on the soil surface was explained by the accumulation of organic residues (animal and vegetal) with consequent decomposition and stabilization of carbon in the soil .…”

“…Soil attributes have been presented as soil quality indexes (i.e., soil porosity and soil organic carbon) because they can be easily modified by soil management and crop cultivation (Almeida et al, 2019).…”

How Does N Mineral Fertilizer Influence the Crop Residue N Credit?

“…In no-tillage systems, there is an accumulation of crop residues which drives the decomposition kinetics and impacts the dynamic of nutrients in soil with the mineralization and/or immobilization of nutrients. The accumulation of crop residues also increases the soil quality, impacting positively the aggregation, water retention and cation exchange capacity in soil with a direct effect on crop production and the diversity of fauna and flora [1][2][3] . In Mediterranean climate, 4 showed an increasing of soil quality in a no-tillage system during 15 years, represented by a higher content of organic matter and the abundance and diversity of micro-arthropods.…”

“…In Mediterranean climate, 4 showed an increasing of soil quality in a no-tillage system during 15 years, represented by a higher content of organic matter and the abundance and diversity of micro-arthropods. In tropical climate, also is commonly the increase of residues in soil in no-tillage systems, but with a rapid decomposition due to the adequate conditions of temperature and moisture to biological activities 2,5 .…”

“…The efficiency of N credit is variable due to the direct influence of biological activity in N mineralization. Studies of 2,14,15 showed that the rate of residue mineralization in soil depends of temperature, soil moisture, and management (i.e. rate, quality/source, and incorporation) of crop residues.…”