Nitrogen budgets in croplands of three counties with different land-use
changes over the past decades
Abstract
Since 1990s, dramatic land-use changes have occurred across mainland
China. Large areas of cereal lands have been converted into
horticultural crops because of lucrative economic benefits. Fruits and
vegetables together in China have consumed more than 30% of N
fertilizers. Therefore, understanding the long-term effects of land use
change from cereals to orchard on N budgets in croplands at the county
scale over long-term is very important for managing N in agricultural
systems in China. We selected three Counties (Luochuan (LC), Mixian
(MX), and Wugong (WG)) on the Loess Plateau with different land-use
changes since 1990s in Shaanxi to compare the changes of N budgets in
croplands at the county scales. The main crops in the three Counties
were cereals (wheat and corn) before 1990s. After 1990s, the land uses
in LC and MX changed dramatically; and LC and MX become the main apple
and kiwifruit production county in China, respectively. The main crops
in WG are still wheat and corn. It provides an ideal reference to
compare the effects of land use changes on N budgets in croplands at the
county scale. The N inputs and outputs, N surplus and N use efficiency
(NUE, computed as N in harvested crop products divided by N inputs) at
the three Counties from 1990 to 2017 were quantified. The annual N
inputs and N surplus of the three Counties since 1990s were increased,
and NUE decreased significantly. Compared to WG, the annual N input and
N surplus of cropland of LC and MX were very high, and NUE was very low.
For example, NUE in LC decreased from 64% in 1990 to 12% in 2017; and
NUE in WG decreased from 55% in 1992 to 38% in 2017. To understand the
fate of surplus N in cropland of LC, we also collected soil profile
samples (0-6 m) from cereal lands and apple orchards in different sites
of the county. The average nitrate accumulation in 0–6 m soil profiles
reached 5611 kg N/ha in 2017. Approximately 67% of the total N
surpluses in LC from 1990 to 2017 was accumulated in soil profile as
nitrate. Land use change from cereals to orchard result in high surplus
N in croplands at the county scale. The nitrate accumulation in the
vadose zone is the main fate of surplus N in the intensive agricultural
landscape, which should be considered an important component of the soil
N budget to optimize production and environmental protection.