Continuous Wheat in Semiarid Regions Long-term Effects on Stock and Quality of Soil Organic Carbon

Continuous wheat (Triticum aestivum L.) cropping in semiarid regions results in variable dry matter production. As a consequence, the balance of soil organic carbon (SOC) may vary across time. The aim of this research was to assess the dynamics and long-term changes of physically and chemically extracted SOC fractions. Soil samples (0- to 5-, 5- to 10-, and 10- to 20-cm depths) from continuous wheat with (f) and without (nf) fertilizer (N + P) under conventional tillage ((CT) for 25 years) and no-tillage ((NT) for 6 years) were taken during the experiment. Mineral-associated ((MOC) 0-0.053 mm), fine particulate ((POCf) 0.053-0.100 mm), and coarse particulate ((POCc) 0.1-2.0 mm) SOC and humic substances were obtained. The SOC variability depended on water availability during fallowperiods (SOC decomposition) or crop cycles (dry matter production). The mean wheat yields were 1.33 (nf) and 2.09 (f) Mg grain ha(-1), with an estimated carbon input of 1.64 (nf) and 2.20 (f) Mg C ha(-1) year(-1). Losses from the initial level were higher in labile fractions, POCc (-75%) and POCf (-53%), than in MOC (-15%). Humic acids present slight differences in their structure and quantity as a result of long-term cropping. Conversion from CT to NT resulted in contrasting results. For an equivalent soil mass, fertilizer application increased SOC by 4.31 Mg ha(-1) (under CT) and 7.29 Mg ha(-1) (underNT). The SOC turned out to be higher under NTwith fertilizer use and lower without application. No-tillage does not increase SOC content by itself; it must be combined with other agricultural practices such as fertilization and/or crop rotation.