Long-Term Effects of Potassium Fertilization and Wheat Straw Return on Cropland Soil Fertility and Microorganisms in the Qinghai–Tibet Plateau

Imbalanced fertilization and inadequate of straw return have led to soil potassium (K) depletion and fertility decline in croplands of the Qinghai–Tibet Plateau. However, how these changes affect soil microbial communities remains unclear. We analyzed soil properties, microbial biomass carbon and nitrogen, enzyme activities, and microbial communities in a long-term field experiment initiated in 1992 with K fertilization and varying wheat straw return amounts. Long-term K application significantly reduced microbial biomass carbon (MBC) and bacterial species richness and inhibited positive bacteria–fungi interactions. By contrast, long-term straw return not only enhanced soil organic carbon (SOC), total nitrogen (TN), available phosphorus (AP), and available potassium (AK) but also increased MBC, microbial biomass nitrogen (MBN), enzyme activities, and fungal species richness. Furthermore, it restructured microbial community composition and improved the stability and connectivity of microbial co-occurrence networks. Critically, these benefits did not scale linearly with straw input quantity. Excessive straw return (>5000 kg ha−1) reduced beneficial effects, destabilized microbial interactions, and elevated risks associated with parasitic protozoa. Balanced potassium application with moderate straw return (approximately 5000 kg ha−1) enhances soil fertility and promotes positive microbial effects in croplands on the Qinghai–Tibet Plateau.