A Farmer Has a Large Field That She Uses to Grow

A farmer has a large field that she uses to grow wheat with minimal agricultural inputs. The wheat grown in this field originally gave a reasonable yield but over time the yield has steadily declined until it becomes economically unviable. Frustrated, the farmer abandons growing wheat in the field and the following year instead plants lentils and produces a good harvest. The farmer grows lentils again the following year and produces another good yield, but the following year she in unable to source lentil seed stock in time to sow the field. Rather than leave the field fallow, she decides to cut her losses and sow wheat again. To her surprise, despite identical climatic conditions and uniform disease conditions, the wheat harvest noticeably improved. What could explain this observation?

A) Thunderstorms have converted atmospheric nitrogen into oxides which have been deposited in the soil by rain and accumulated.
B) Lentils secrete an exotoxin which has killed soil bacteria, resulting in less competition for nutrition.
C) Wheat stubble had built up in the soil of the field, inhibiting the growth and development of future wheat plants. Two years of lentils gave enough time for decomposing microorganisms to break down the build-up of stubble.
D) Lentils are a leguminous plant, therefore they have increased the available nitrogen levels in the field.
E) Lentils use fewer nutrients than wheat, meaning there was more available for the wheat when it was grown after two years of growing lentils.