Temporal and spatial distribution and development of permanent gully in cropland in the rolling hill region (phaeozems area) of northeast China

The cropland in the Songnen phaeozems area (SPA) experiences an extensive gully erosion in the black soil region of northeast China. This study investigates the historical trends, current status, and future trajectories of permanent gully (PG) development, aiming to reveal the mechanisms behind PG development and protect the black soil. We selected nine study areas, totalling 378 km2, across three latitudinal zones in the SPA with slopes of 0-1 %, 1-3 %, and 3-5 %. The results revealed a consistent rise in gully density over time in the SPA, with values increasing from 0.87 km/km2 in 1970 to 0.94 km/km2 in 2010, and 1.32 km/km2 in 2021. Conversely, the ground lacerative degree significantly decreased from 0.025 km2/ km2 in 1970 to 0.010 km2/ km2 in 2010, and rebounding slightly to 0.019 km2/ km2 in 2021. The development rate of PG in the northern area was higher than that in the middle and southern areas, with steeper slopes (3-5 %) having more PGs compared to gentler slopes. The average rate of gully area extension increased with increasing gully level. From 1970 to 2010, the area of PG experienced a decline attributed to extensive gully management. However, subsequent decade (2010-2021) experienced a more rapid PG development, resulting in an annual cropland loss of 0.097 %. The northern area has consistently exhibited the fastest PG development rate in 1970-2021. It is noteworthy that managing gullies through reclamation after filling PG has resulted in the re-generation of secondary gullies in the same locations, which developed more rapidly than natural gullies. The reclamation after filling PG, without improvement of field drainage systems, contributes to an accelerated development rate of PG in cropland. Despite significant reductions in the total area and length of PGs through large-scale gully management, the notable increase in the development rate of gullies warrantsattention in the SPA of northeast China. Furthermore, the global community should take caution with inappropriate gully management measures because they not only prove ineffective but aggravate gully erosion.