Acidic soil Europe Helianthus annuus L. N uptake N use efficiency SDS-PAGE analysis SOC and STN stocks Texas High Plains Triticum aestivum L. WHCNS Zea mais L. agrobiodiversity agroecology catch crop cereal rye complexity conservation conservation agriculture conventionalization cover crop cover crops crop residue incorporation crop rotation cropping system design cropping systems double cropping durum wheat economics environmental quality enzyme activities