Tensile strength of mollisols of contrasting texture under influence of plant growth and crop residues addition

Tensile strength (TS) of soil aggregates is considered a sensitive and important indicator of the effects of the management practices on soil structure quality, which affects the seed germination and the initial crop growth. However, the influence of plant growth, crop residues addition and the produced aggregating agents on TS has not been widely studied. The objectives of this study were: i) to determine the specific effects of plant growth and different types, rates and location of crop residues in the aggregates tensile strength, and ii) to assess the relationship between the aggregating agents produced by plant growth and crop residues addition and the aggregate tensile strength of soils of contrasting texture. A greenhouse experiment was carried out in pots with a loamy soil (Typic Hapludoll, Santa Isabel series) and a silty-loamy soil (Typic Argiudoll, Esperanza series) under controlled conditions for 112 days. For each soil the following treatments were set up in triplicate: (i) soil type, (ii) with or without plant growth of wheat (Triticum aestivum L.), (iii) with or without residues addition, (iv) location of residues (surface vs. incorporated), (v) wheat vs. soybean (Glycine max L.) residues, and (vi) residue rates (6.3 and 15.7 g of dry matter per pot for wheat, and 6.3 and 18.8 g of dry matter per pot for soybean). Pots were exposed to wetting and drying (W/D) cycles. TS values and aggregating agent's content, i.e., total organic carbon (TOC), particulate organic carbon (POC), hot water extractable carbohydrates (HWEC), dilute acid extractable carbohydrates (DAC), total carbohydrates (TC), total glomalin-related soil protein (T-GRSP), and easily extractable glomalin-related soil protein (EE-GRSP) were measured. TS were significantly lower in the Typic Hapludoll (TS = 39.9 kPa) than in the Typic Argiudoll (TS = 61.6 kPa). TS values were significantly higher in the treatments with plants of wheat than in treatments without plants (49.5 vs. 30.3 kPa in the Hapludoll and 71.2 vs. 50.9 kPa in the Argiudoll). Plant growth increased TS through physical mechanisms, i.e. a greater number of drying-wetting cycles. Also, plant growth increased TS by producing aggregating agents. TS values were not directly affected by the addition of different types, rates and locations of crop residues. However, they increased the content of aggregating agents. Multiple regression analysis showed that TS was significantly related to soil type, TC and T-GRSP. TS increased with increasing TC and T-GRSP. These two variables explained 87% of the model variation. The obtained model provides a basis for understanding which are the most important aggregating agents and, consequently, which are the better management systems to improve o recover the structure quality of soils with different textures.