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There are concerns that restored grasslands currently under the Conservation Reserve Program (CRP) will experience increased soil erosion when they are returned to crop production. Our objective was to compare runoff, erosion, and erodibility on CRP land converted to annual hay production (permanent hayed, PH) and crop production under conventional-till (preplant disk tillage, CT) and no-till (NT) management. Erosion study was conducted in central North Dakota on Typic Argiustoll soil 6 yr after the CRP land had been converted to hay production and crop production with a spring wheat (Triticum aestivum L.)--winter wheat-dry pea (Pisum sativum L.) rotation. Runoff volumes and soil loss (by alum precipitation of sediment) were measured on 1.5 by 5 m bordered plots on 4% slope under a rainfall simulator delivering 1-to 3-h rains at 50 or 75 mm [h.sup.-1], followed by three rains of 20 min or less at rates from 25 to 125 mm [h.sup.-1.] Erodibility was calculated from ratios and regressions of soil loss rates versus runoff rates measured at relative steady state. Runoff rates from 50 and 75 mm [h.sup.-1] rains for CT, NT, and PH averaged 9, 12, and 21 mm [h.sup.-1], respectively, and supported soil loss rates of 20, 7, and 8 g [m.sup.-2] [h.sup.-1]. Erodibility of undisturbed CT, NT, and PH was 1.65, 0.29, and 0.28 g [m.sup.-2] [mm.sup.-1], respectively, showing NT did not differ from PH and that CT management increased erodibility six-fold above PH. Thorough disk tillage increased erodibility three-fold over CT, 15-fold over NT, and nine-fold over PH. Complete, nondisturbing residue removal increased erodibility less than tillage, from 1.2 times for CT to 2.5 times for NT. Chemically weeded NT exhibited the same low erodibility as the grassland PH treatment under the conditions of study--4% land slope, above average precipitation, and a residue-productive crop rotation. However, erodibility of tilled NT was significantly higher than that of tilled PH, showing the higher inherent stability of grassland surface soil with its perennial plant rant structures.

During rainfall events, runoff water carries sediment downslope. It has been hypothesized that the detachment potential of the runoff water would decrease as the sediment content is increased until a sediment transport capacity, [T.sub.c], is reached and the excessive amount of sediment beyond [T.sub.c] would then be deposited. Translating this concept to a hill slope setting implies that water and sediment from upslope areas would affect erosion processes on a downslope segment. However, there are few experimental data to support this hypothesis or the run-on water and sediment transport capacity effect. A dual-box system, consisting of a 1.8-m-long sediment feeder box and a 5-m-long test box, was used to quantify run-on water and sediment effects. The test box has holes at the bottom for controlling the vertical hydraulic gradient. Experimental variables were rainfall intensity, slope steepness, near-surface hydraulic gradient, and run-on sediment concentration. Results demonstrated that under free soil drainage conditions, a decrease in run-on sediment content caused a corresponding increase in downslope sediment detachment, resulting in a relatively constant sediment delivery from the test box. Under artesian seepage conditions, sediment deliveries were three to six times greater than those under drainage conditions and decreased as the erosion event progressed, possibly because of a reduction in soil erodibility. Sediment detachment by the run-on water increased as either slope or rainfall intensity was increased or when the surface was changed from a drainage to a seepage condition. These results demonstrated that the sediment delivery reached a dynamic equilibrium and the dual-box system can be used to study erosion processes similar to those occurring on a hill slope. A process-based erosion model needs to account for surface hydraulic conditions and the associated erosion processes and their interactions.

Understanding the effects of upslope runoff and soil pipe collapse on slope water erosion can provide scien-tific basis for preventing Mollisol degradation caused by soil erosion. We conducted an experiment to investigate the effects of upslope inflow rate and soil pipe collapse on slope water erosion and to quantify the contribution of soil pipe erosion to slope soil erosion. The experiment included three inflow rates (30, 40, and 50 L·min -1 ) and three near-surface soil hydrological conditions (without soil pipe: NP; with soil pipe but no pipe flow: PF 0 ; with pipe flow: PF 1 ). The results showed that: 1) Slope soil erosion increased with increasing inflow rates; when the inflow rate increased from 30 L·min -1 to 40 and 50 L·min -1 , slope soil erosion increased by 100.0%-111.5% and 214.8%-289.2%, respectively. 2) The soil pipe occurrence and pipe flow formation aggravated the slope water erosion process. At inflow rates of 30, 40, and 50 L·min -1 , slope soil loss under the PF 0 and PF 1 treatments were 1.4-1.6 times and 1.7-2.1 times of that under the NP treatment. The contribution of soil pipe erosion to slope soil loss was 26.7%-37.6% under the PF 0 treatment and 40.5%-51.9% under the PF 1 treatment. 3) Soil pipe collapse intensified the rill erosion process. Compared with the NP treatment at 30, 40, and 50 L·min -1 inflow rate, rill erosion amounts under the PF 0 and PF 1 treatments increased by 38.1%-66.0% and by 93.7%-128.4%, respectively. Our results suggested that increasing upslope inflow rate resulted in higher surface runoff velocity, which promoted runoff detachment and transport capacity, and then aggrandized the amount of slope soil erosion. Moreover, soil pipe collapse exacerbated rill erosion process. When the soil pipe collapsed, all surface runoff was converted to soil pipe flow, which accelerated flow velocity and slope soil erosion process, and then increased the amount of slope soil erosion.

Research on the processes and mechanisms of compound soil erosion by multiple forces can provide scientific guidance for precisely controlling cropland soil erosion. Based on the seasonal alternation of freezing-thawing, snowmelt, wind, and rainfall erosion forces on sloping farmlands under natural conditions from November to next October of each year, we used a set of indoor simulation experiments of multi-force superimpositions to analyze the compound soil erosion processes of snowmelt (1 and 2 L·min -1 ), wind (12 m·s -1 ), and rainfall (100 mm·h -1 ). We further discussed the erosion effects of multi-force superimpositions. The results showed that, under single snowmelt erosion, an increase in snowmelt flow had a greater effect on sloping snowmelt erosion intensity than that of sloping runoff rate. When sloping snowmelt flow increased from 1 L·min -1 to 2 L·min -1 , sloping runoff rate and erosion intensity increased by 2.7 and 4.0 times, respectively. Under snowmelt-wind superimposition erosion, previous sloping snowmelt erosion inhibited late wind erosion occurrence. As sloping snowmelt flow increased from 1 L·min -1 to 2 L·min -1 , the inhibiting action subsequently increased and wind erosion intensity caused by previous snowmelt reduced by more than 50%. Both wind erosion and snowmelt-wind superimposed erosion intensified late rainfall erosion. The early wind erosion increased rainfall erosion by 24.5%. The snowmelt-wind superimposed effect increased the later slope rainfall erosion by 132.8% and 465.4% under 1 and 2 L·min -1 snowmelt runoff rates, respectively. The compound soil erosion amount driven by multiple force superimposition was not the sum of the corresponding erosion amount caused by single erosion force, with promoting or inhibiting effects of erosion force superimposition. The erosion effect of snowmelt-wind superposition was negative, but that of wind-rainfall superposition and snowmelt-wind-rainfall superpositions were positive.

Understanding the responses of soil enzyme activities to elevated CO 2 concentration and warming can provide a scientific basis for nutrient management of croplands under global climate change. We conducted a pot expe-riment with climate chamber to examine the effects of elevated CO 2 concentration and warming and winter wheat growth on soil enzyme activities. There were four climate scenarios: control (CK, 400 μmol·mol -1 CO 2 concentration+normal ambient temperature), and CO 2 concentration elevation (ECO 2 , 800 μmol·mol -1 CO 2 concentration+normal ambient temperature), elevated temperature (ET, 400 μmol·mol -1 + temperature increased 4 ℃), and elevated CO 2 concentration and temperature (ECO 2 +T, 800 μmol·mol -1 CO 2 concentration + temperature increased 4 ℃). We measured the activities of soil β-glucosidase (βG), β-N-acetyl glucosidase (NAG), alkaline phosphate (ALP) and polyphenol oxidase (PPO) at four growth stages (JS, jointing stage; AS, anthesis stage; FS, filling stage and MS, maturity stage), with and without winter wheat planting. Without winter wheat planting, there was no significant difference in four kinds of soil enzyme activities between ECO 2 and CK, while ET and ECO 2 +T treatments had significant negative effect on soil enzyme activities. With winter wheat planting, compared with CK, ECO 2 and ECO 2 +T treatments did not affect the activities of those four soil enzyme; but the ET treatment had great impact on soil ALP and PPO activities. The activities of four kinds of soil enzyme were significantly diffe-rent between the ET and ECO 2 +T treatments. Compared with ET treatment, ECO 2 +T treatment increased soil βG activity at the JS, decreased NAG activity at the JS, increased ALP activity at both AS and FS, decreased PPO activity in the JS and increased in the AS. The interaction of elevated CO 2 concentration and warming had significant effect on soil NAG and ALP activities with and without winter wheat planting. The interaction of warming and expe-rimental stage had significant effect on four kinds of soil enzyme activities without winter wheat planting, but the interaction of warming and crop growth stage had significant effect on ALP and PPO activities with winter wheat planting. The interaction of elevated CO 2 concentration, warming and experimental period had significant effect on soil βG, ALP and PPO activities without winter wheat growth, while with winter wheat growth, it had significant impact on NAG, ALP and PPO activities. The winter wheat growth had significantly inhibitory effect on βG, NAG and ALP activities in the two early growth periods (JS+AS), significant promoting effect in the later growth periods (FS+MS), and significantly inhibitory effect on PPO activity during whole growth period. Overall, elevated CO 2 concentration did not affect soil enzyme activities, while the elevation of CO 2 concentration and temperature on soil enzyme activities differed among the soil enzymes at different growth stages. In addition, the responses of four soil enzyme activities to the interaction of elevated CO 2 concentration and warming varied with and without winter wheat planting.

Revealing the responses of soil microbial community to soil erosion can provide guidance for agricultural ecosystem management. We investigated the impacts of soil erosion intensity on soil microbial quantity at the Binzhou River Basin, a typical thin layer Mollisol area in Bin County of Heilongjiang Province, using 137 Cs tracer. The results showed that there were seasonal variations in soil microbial quantity. The abundance of soil microbes in summer was considerably higher than that in autumn. Bacteria was more sensitive to seasonal variation than fungi and actinomycetes, which was increased by 1.4-2.6 times and 1.4-2.2 times in summer compared with autumn in different parts of slope and watershed, respectively. The spatial variation of soil erosion intensity had an important effect on the spatial distribution of soil microbial community. The highest proportion of bacteria was found at lower deposition area of slope (84.4%) and at the lightly eroded area of the downstream (85.4%). The numbers of soil microbes, soil bacteria and actinomycetes were negative linearly correlated with soil erosion modulus, with correlation coefficients of -0.595, -0.554 and -0.291, respectively. Soil erosion and deposition induced spatial differences in soil physical and chemical properties, with consequences on spatial distribution of soil microbial community.

Snowmelt erosion is an important way of soil loss in Chinese Mollisol region. However, little is known about the effects of seepage flow and soil thaw depth on hillslope snowmelt runoff erosion. An indoor simulated experiment was conducted to analyze the impacts of seepage flow and soil thaw depth on hillslope snowmelt erosion. There were two snowmelt flow rates (1 and 4 L·min -1 ), two soil thaw depths (5 and 10 cm), and two near-surface hydrological conditions (with and without seepage flow). The results showed that hillslope runoff depth and soil erosion amount in the treatment with seepage flow were 1.1 to 1.2 times and 1.3 to 1.9 times of those in the treatment without seepage flow, respectively. Under two snowmelt flow rates, when soil thaw depth increased from 5 cm to 10 cm, hillslope runoff depth and soil erosion amount increased by 10.0% to 13.5% and 15.4% to 37.1% in the treatment without seepage flow, respectively. In the treatment with seepage flow, when soil thaw depth shifted from 5 cm to 10 cm, hillslope runoff depth increased by 6.5% to 8.5%, and soil erosion amount remained stable. Moreover, hillslope rill development was comprehensively influenced by seepage flow, soil thaw depth, and snowmelt flow rate, with rill erosion amount occupying more than 72% of hillslope snowmelt erosion amount. Compared with the treatment without seepage flow, flow velocity and shear stress under the treatment with seepage flow increased by 20.3% to 23.2% and 37.0% to 51.3%, respectively; but Darcy-Weisbach friction coefficient reduced by 9.0% to 21.4%, which caused an increase of hillslope snowmelt erosion. In addition, seepage flow enhanced rill development, which caused rill erosion amount to increase by 43.6% to 69.9% compared with the treatment without seepage flow, and it further resulted in the increase of hillslope snowmelt erosion amount. The main reason for soil thaw depth enhancing hillslope snowmelt erosion amount under the treatment without seepage flow was that both sloping runoff erosivity and erodible materials increased with increasing soil thaw depth. Furthermore, soil thaw depth had a significant impact on hillslope rill morphology development under the treatment with seepage flow. Rill widening process was dominated when soil thaw depth was 5 cm, whereas rill incision process was dominant when soil thaw depth was 10 cm. This study could improve the understanding of hillslope snowmelt erosion mechanism in Chinese Mollisol region and provide theoretical guidance for the development of water erosion model.

Agricultural non-point source pollution is one of severe problems for water environment of agricultural area in China. The effects of near-surface soil water conditions on agricultural non-point source pollutant (AGNSP) transport during soil erosion processes, especially antecedent soil moisture was saturated, was developed by using artificial simulation rainfall experiment. The results showed that antecedent soil water content had great impact on AGNSP transport during soil erosive processes. Under the same soil texture, the AGNSP concentration and loading with runoff and sediment when the antecedent soil water content was saturated were greater than that of soil moisture un-saturated condition, and they would be increased as antecedent soil moisture increased. The approach of soil nitrogen loss was rainfall runoff; nitrogen loss with runoff was about 90.4% to 99.8% of total loss. The approaches of soil phosphorus were runoff and soil loss (sediment), the loss with runoff was about 2.67% to 23.5%, and the loss with sediment was about 76.5% to 97.3%. Soil texture had great influence on soil phosphorus loss; the concentration and loading of dissolved phosphorus (DP) with sediment from Yangling Loutu were greater than that of Ansai Loess. Some pertinence suggestions were given to control agricultural non-point source pollution, such as the best management practices.