Conservation Tillage and Nitrogen Application Promote Soil Carbon and Nitrogen Mineralization and Improve Maize Photosynthetic Characteristics and Yield

Exploring the effects of conservation tillage and nitrogen application on soil carbon and nitrogen mineralization characteristics, as well as their impact mechanisms on maize photosynthetic characteristics and yield, in order to provide a basis for improving soil fertility and promoting maize production in arid areas of Northwest China.

This study conducted a two-year (2019-2020) maize field location experiment, with traditional tillage (CT) as the control, and set up three protective tillage measures (no tillage: NT; no tillage in wheat season and rotary tillage in maize season: OT; ridge cultivation with no tillage: RNT) and two nitrogen application levels (N0: 0; N2: 170 kg N·hm^-2), for a total of 6 treatments. The effects of conservation tillage and nitrogen application on soil nutrient content, carbon and nitrogen mineralization characteristics, maize photosynthetic physiological characteristics, and yield were investigated systematically.

Conservation tillage and nitrogen application significantly increased soil nutrient content (P<0.05). Under N2 level, compared with CT treatment, NT, OT, and RNT treatments increased soil organic carbon (SOC), microbial biomass carbon (MBC), total nitrogen (TN), and microbial biomass nitrogen (MBN) content by 8.6%-24.7%, 18.9%-27.0%, 8.9%-20.2%, and 0.3%-24.9%, respectively. The application of nitrogen significantly increased the accumulation of soil carbon mineralization (C_min), nitrogen mineralization accumulation (N_min), and their mineralization rates. Conservation tillage further improved the soil carbon and nitrogen mineralization characteristics. C_min and N_min reached their maximum values under RNTN2 and OTN2 treatments, respectively, which increased by 4.0%-30.2% and 8.0%-52.4% compared with other treatments. Conservation tillage and nitrogen application significantly increased the net photosynthetic rate (Pn), transpiration rate (Tr), and stomatal conductance (Gs) of maize leaves (P<0.05). The Pn, Tr, and Gs of maize reached their maximum values in the OTN2 treatment for two years. The yield of maize in two years showed the order of OTN2>RNTN2>NTN2>CTN2>NTN0>CTN0, with the highest yields of 10.52 and 10.91 t·hm^-2, respectively, which increased by an average of 24.5% and 27.5% compared with other treatments. Based on structural equation modeling analysis, it was found that conservation tillage mainly increased soil nutrient content, promoted soil organic carbon and nitrogen mineralization, enhanced soil available nitrogen supply capacity, and thus promoted the enhancement of maize photosynthetic capacity, achieving maize yield increase.

In the arid northwest region, conservation tillage and nitrogen application were of great significance in promoting soil carbon and nitrogen mineralization, increasing maize yield, and maintaining soil productivity. It was recommended that no tillage in wheat season and rotary tillage in maize season combined with nitrogen application was the optimal management measures for increasing maize yield and efficiency.