It has been shown that the joint application of phosphorus (P) and ammonium (N-NH⁺₄) increases maize root proliferation and P acquisition by maize in alkaline soils, but this has not been shown in acidic soils for legumes. A greenhouse experiment was conducted to assess the effect of the joint application of P and NH₄⁺ on soybean root growth and P acquisition. Soybean was grown in glass-walled pots without P, with monoammonium phosphate (MAP) and triple super phosphate (TSP) applied on the soil surface or localized. The soil P increased irrespective of the P source and localization. The rhizosphere pH was decreased by MAP, while the soil bulk pH was not affected. The TSP increased the root length by 55% and MAP by 76% over the control, and the number of root tips increased by 21% with TSP, 58% with MAP applied on the soil surface, and 78% with MAP localized. The soybean dry matter, N and P uptake, and P use efficiency were increased by P fertilization, mainly with MAP localized. The joint application of P and ammonium decreases the soybean rhizosphere pH, which results in root proliferation early in the cycle, and eventually in higher P uptake and use efficiency.

Intercropping maize along with forages fertilized with N can potentially increase soil carbon sequestration, contributing to climate change mitigation. However, there is a lack of knowledge if the input of new C sources in this production system impacts the cycling of the original soil C and SOM fractions, especially in tropical soils. To investigate this, soil samples were taken up to 80 cm depth from a 7-year experiment where ruzigrass (Urochloa ruziziensis), palisadegrass (Urochloa brizantha) and Guinea grass (Megathyrsus maximus) were intercropped with maize fertilized with (270 kg N ha-1) or without N. In these samples, SOM was fractionated by size into particulate (POM) and mineral-associated (MAOM) organic matter and submitted to 13C natural abundance measurements. Intercropping with Guinea grass reduced the δ13C values in comparison to ruzigrass and palisadegrass, especially under N fertilization. Forage grasses reduced the δ13C values up to 40cm, indicating the contribution of the grasses for the cycling of the original carbon of the soil. Nitrogen supply increased the contribution of C from the grasses to the POM fraction if compared to the no N application. Further, 13C in POM at 0-10 and 10-20 cm differed from deeper layers, probably due the above- and belowground C inputs on the uppermost soil layers. Under N supply, Guinea grass lowered the δ13C value, which did not occur in the palisade and ruzigrass treatments. In contrast to POM, the δ13C values of MAOM decreased in all depths, with the highest change at the uppermost soil layer. Our findings showed that intercropping influenced the cycling of total C and SOM fractions , with differences in the soil profile. However, only Guinea grass changed δ13C values under N supply.

Liming is essential for achieving high crop yields in tropical acidic soils, but lime dissolution releases CO ₂ , whereas silicate slag does not, while gypsum improves subsoil fertility. Although the benefits of these amendments for soil fertility are well known, their combined effects on CO ₂ emissions and soil carbon (C) accumulation in cropping systems remain unclear. We evaluated the effects of lime and silicate applied alone or in combination with gypsum, in a soybean ( Glycine max L.) double‐cropped with maize ( Zea mays L.) intercropped with Urochloa ruziziensis system under no‐till (NT) and conventional tillage (CT), in two side‐by‐side field experiments. The experiments were conducted on a Typic Haplorthox in Southwestern Brazil over 2 years. Treatments included (i) lime, (ii) silicate slag, (iii) lime + gypsum, (iv) silicate slag + gypsum, and (v) control. Root and shoot dry matter, plant carbon (C) input, CO ₂ emissions, and soil C stocks were measured in the second‐year soybean season. The application of soil amendments increased shoot and root dry matter and C inputs under NT and CT but did not alter total C stocks across the soil profile. Cumulative CO ₂ emissions were not affected by soil amendments under NT; however, emissions increased with lime + gypsum under CT. Alleviating soil acidity in tropical cropping systems through the application of lime, silicate, and their combination with gypsum can enhance crop yields while reducing yield‐scaled (normalized) CO ₂ emissions.