To detect soil changes related to vegetation and fertility restoration in a long fallow agricultural system of the Venezuelan Andes, 32 soil (A horizon, 0 -15 cm depth) and litter characteristics were studied in plots at different stages of the fallow-cultivation cycle. Four sectors of the valley were sampled, each one including seven plots: recently ploughed after a long fallow period; 1 and 2 years under potato crop; 1, 4 -5 and 8 years in fallow and natural vegetation. Each sector had similar topography, parent material and exposure to reduce the spatial heterogeneity that can hinder the synchronic analysis of the succession. Data of each sector were standardized before the statistics comparisons. Although all soils were acidic, those involved in the cropping cycle, or with only 1 year of fallow, had a significantly lower pH than the others, indicating that soil cultivation triggered off acidifying processes intense enough to overcome the strong buffering indices of the soils. These acidifying processes, facilitated by the acidity generating ions that widely dominate the desaturated CEC, are surely due to the stimulation of litter and soil organic matter mineralization after ploughing the soil and also to the N fertilization of the cultivated soils. The high soil contents of exchangeable Al 3þ and free Al oxides suggest that Al plays an important role in SOM stabilization, lowering its mineralization. No successional increase of any main plant nutrient was found in soil or litter. Moreover, soil available P and litter-P contents are higher during the cultivation phase and at the beginning of the succession, probably as a consequence of fertilization. Water holding capacity was similar for all soils, indicating that this long fallow agriculture system does not change the water storage capability of the soil, an important aspect for the role of the páramo in the regional water balance. The main characteristics of soils and litters, including their d 13 C values, were useless to monitor soil changes during cultivation and fallowing. Nevertheless, the d15 N values of SOM decreased steadily along the crop-fallow chronosequence, while those of litterfall were rather constant from recently ploughed to 1 year fallow soils, decreasing suddenly on medium fallow plots and again on the virgin páramo. Together, the d15 N values of soil-and litter-N grouped the soils following the crop-fallow chronosequence, suggesting that a change from 'open' to 'closed' N cycling is the characteristic that better discriminates the soils along the succession. q
A biological (3 and 6 weeks aerobic incubation) and a chemical method [successive extractions with cold 0.1 (H1-N) and 0.5M HCl (H2-N)] were applied to 21 soils to determine: a) the potentially mineralizable-N; b) the most useful soil variables for predicting soil-N availability; and c) their usefulness for predicting N uptake by a greenhouse wheat crop. At t= 3, both net N mineralized (NNM) and net N mineralization rate (NNMR) were correlated: a) positively with SOM-and CEC-related variables; and b) positively with soil δ 15 N and negatively with soil pH, suggesting that N-mineralization, dominated by nitrification, is associated with NO 3 −
This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues.Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. The role of the native legume Lupinus meridanus on fertility restoration in long fallow agriculture of the Venezuelan Andes was analyzed using field and laboratory data and a process-based model of succession (FAPROM). L. meridanus biomass was estimated in 117 plots with 1-12 years in fallow and N 2 fixation was calculated using the natural 15 N abundance. Additionally, the legume was sowed at the beginning of the fallow period in three plots and, after two years, its effect on a posterior potato crop was evaluated. FAPROM was used to simulate L. meridanus development and N 2 fixation during the fallow period in competition with other five species; after the early introduction of the legume; and in L. meridanus monoculture. High biomass variability between plots of the same age was observed in the field (coefficients of variability exceeding 100%). In consequence, the estimated N 2 fixation was also variable, increasing during the fallow period to attain a maximum after five years (1.36 ± 2.84 g N m −2 year −1 ) and decreasing afterwards. Cumulative fixation after 10 years in fallow was estimated in 4 g m −2 , a modest contribution to fertility restoration. Nevertheless, this cumulative value rose to 23.5 g m −2 when the potential fixation was estimated considering the plot of each fallow age where L. meridanus attained its highest biomass. FAPROM was modified to include two stochastic sources of variability, in initial soil organic N and in the probability of colonization of the different species, and succeeded to reproduce mean values and the variability in L. meridanus biomass and N 2 fixation. Model and experimental data indicate that the early introduction of the legume is a promising strategy to improve potato production and shorten the fallow period. This species has a high potential to fix N 2 but its performance is hampered by the low probability of plot colonization.
Background: Chemical methods allowing a single soil extraction followed by multi‐elemental simultaneous measurement by ICP‐OES are increasingly used to predict plant uptake; however, calibration results against crop response are scarce and contradictory.Aims: Our aims were to evaluate the efficacy of five extractants to predict nutrient uptake by a greenhouse wheat crop, as well as the influence of soil properties on nutrient concentrations in soil extracts and wheat plants.Methods: Unlike other calibration studies, we monitored the pre‐seeding to post‐harvesting changes in soil available Ca, K, Mg, Cu, Fe, Mn, Zn, and Al. We extracted 14 acidic soils (C content: 47–114 g kg−1) with two traditional (AA: ammonium acetate; DTPA: diethylenetriamine‐pentaacetic acid) and three multi‐element extractants (AB‐DTPA: ammonium bicarbonate‐DTPA; Mehlich‐3; AA‐DTPA: ammonium acetate‐DTPA).Results: Relationships between bioavailable and chemically extractable elements were strong for K (R2 = 0.776 to R2 = 0.882; p < 0.001) and Zn (R2 = 0.663 to R2 = 0.721; p < 0.001), especially for AB‐DTPA and AA‐DTPA. Multiple regressions including also soil properties can predict wheat‐Ca (Feoxihydroxides, clay and CaAB‐DTPA; R2 = 0.656; p < 0.001) and wheat‐Cu [Aloxihydroxides and either CuAB‐DTPA (R2 = 0.515; p < 0.01) or CuAA‐DTPA (R2 = 0.472; p < 0.01)]. Pre‐seeding to post‐harvesting changes in KAA‐DTPA and KAB‐DTPA were strongly related with K uptake by wheat (R2 = 0.927 and R2 = 0.949, respectively; p < 0.001); similarly, for wheat‐Zn the best relationships were with ZnMehlich‐3 and ZnAA‐DTPA (R2 = 0.654 and R2 = 0.757, respectively; p < 0.001).Conclusion: Consequently, chemical extractants alone can adequately predict K and Zn bio‐availability, and combined with some soil properties can predict wheat uptake of Ca and Cu, but not that of other nutrients.
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