ORGANIC C DYNAMICS IN GRASSLAND SOILS. 2. MODEL VALIDATION AND SIMULATION OF THE LONG-TERM EFFECTS OF CULTIVATION AND RAINFALL EROSION
Canadian Journal of Soil Science
Atomit' Sr'[ence, Wageningen, The Nctherlands; tDtpa.rrmenr ol soil Scie,nce, University'of Saskatt.heu-an, Saskatoon, Sask. S7N 0W0: and*Dep(trtmett of Plant and Soil Bittlogv, (Jniv'ersin' of Calitornia, Berkelev, CaltJ 9472 VonoNEv,R. P.,Ve.NVpt-.s"J. A.ANoP,qur-.E A l98l OrganicCdynamicsin €,fassland soils. 2. Model validation and simulation of long-term cffects of cultiva tion and rainfall erosion. Can J. Soil Sci. 6l: 2ll'224' The amounts of organic matter in native prairie and in an
... irie and in an adjacent cultivated field were compared with the output from a simulation model describing organic mattel dynamlcs. The effects of past and possible future soil management practices, and the loss of organic C through rainfall erosion were incorporated into the simulation study. Seventy vears of cultivation increased the bulk density of the A horizon by an average of I 67o along the catena of a Black Chemozemic soil. Organic C had decrcasedby 367o in the soil proflle at the mid-slope position. Losses of organic N were 5-l0% less. Depletion of organic c and N from the Ah horizon accounted for >90% of the total loss from the soil profile. Ther:efore, extrapolation of data from surface soil, based solely on changcs in the concentration of organic C and N. could result in an overestimation of organic matter losses from soils. Microbial biomass in thc Ap horizon of the cropsummer-fallou site was 3O7o less than in the Ah horizon of the native prairie. The model predicted an imnrediate rise in microbial biomass C upon cultivation of the native prairie due to a large initial input of gfassland litter ancl roots. Subsequcntly. the microbial biomass C decreased and approached a steady-state level which was 25t/c lcss than in the nativc prairie. The model indicates that large quantities ofN released duringthe initial years of culttvation woulcl not have been totally utilized by the cultivated crops, thercfore resulting in major losses to the environment. However, now the organic matter is reaching a steady-state level and only small net release of N can be expccted; extcrnal N sources are required fbr optimum crop production. Management practlces such aS straw removal and cropping sequence have sholt-tefm etfects on the rate ofdepletion of soil orgrLnic C. Similar equilibrium levels of soil organic matter were prcdicted after 100 yr of cr:ltivation in simulation studies that did not consider erosion los ses. The inclusion of rainfall erosion losses indicated that major organic C and other nutrient losses will occur in management practices that include significant portions of tallow in the cropping sequence. Les quantit6s de matidre organique (MO) dans un sol sous prairie originelle et dans un sol voisin sous culture ont 6t6 compar6es avec lcs donn6es mod6lis6es rclatives aux m6canismes de transfbrmation de la MO. Le moddlc mathdmatique prenalt en compte les r:ffets des pratiques agronomiques ant6rieures et fventuelles (futures), ainsi que les d6pr:rditions de C dues d l'6rosion pluviale. Aprds 70 anndcs de culture, la densit6 apprlrente de I'horizon A s'est accrue de 16% en moyenne tout au long de la chainc rpresented at the C. S. S.S. Symposium entitled "Long-term effects of intensive cultivation on soil quality. " Halifax. 19'79. Can J. Soil Sci. 6l:211-224 (May l98l) 211 Can. J. Soil. Sci. Downloaded from pubs.aic.ca by COLORADO STATE UNIV LIBRARIES on 12/24/13 For personal use only.