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Title:   Author  item
                                            Sainju, Upendraitem
                                            Lenssen, Andrewitem
                                            Caesar, Thecanitem
                                            Waddell, Jed
  Submitted to: Soil & Tillage Research
                                    Publication Type: Peer Reviewed Journal
                                    Publication Acceptance Date: 6/16/2006
                                    Publication  Date: N/A
                                    Citation: N/AInterpretive Summary: Soil organic matter is a key indicator of soil quality and productivity but its measurement alone does not adequately reflect changes in soil quality and nutrient status, because it has large pool size and inherent spatial variability. Measurement of biologically active fractions of organic matter, such as microbial biomass C and N (MBC, MBN), potentially C and N mineralization (PCM and PNM), NH4-N, and NO3-N, that change rapidly over time, could better reflect changes in soil quality that alter nutrient dynamics. These fractions can provide an assessment of soil organic matter changes induced by management practices, such as tillage and cropping sequences, which can influence nutrient availability and crop and yields. 

We examined the effects of two tillage practices [conventional till and no-till], five crop rotations [continuous spring wheat (CW), spring wheat-fallow (W-F), spring wheat-lentil (W-L), spring wheat-spring wheat-fallow (W-W-F), and spring wheat-pea-fallow (W-P-F)], and a Conservation Reserve Program (CRP) planting on soil labile C and N pools at 0- to 5- and 5- to 20-cm depths. Each phase of the crop rotation (W-F, F-W, W-L, L-W, W-W-F, W-F-W, F-W-W, W-P-F, P-F-W, and F-W-P) was included in every year. The C and N pools included microbial biomass C and N (MBC and MBN), potential C and N mineralization (PCM and PNM), and NH4-N and NO3-N contents. A field experiment was conducted from 1998 to 2003 in Havre, MT.

The MBC, PCM, and NH4-N contents were not influenced by treatments. The MBN at 0- to 5-cm was greater in P-F-W than in CRP, F-W, and W-W-F in conventional till, but was greater in F-W-W than in W-F, F-W, L-W, and W-F-W in no-till. At 5- to 20-cm, MBN was greater in W-F-W than in other rotations, except in CW and P-F-W, in conventional till. Similarly, PNM at 0- to 5-cm was greater in W-P-F than in other rotations, except in W-L, in conventional till. The NO3-N content at 0- to 5-cm was greater in W-W-F than in other rotations, except in W-F and W-P-F, in no-till. At 5- to 20-cm, NO3-N was greater in W-P-F than in other rotations, except in W-L, in conventional till, but was greater in W-W-F than in CRP, CW, and P-F-W in no-till.

The results indicate that residue quality and quantity, placement in the soil, time since last residue was returned to the soil, and total rate of N fertilization from 1998 to 2003 or recently applied N fertilizer due to tillage and crop rotation treatments influenced soil N mineralization and availability. Residue with higher N concentration or lower C/N ratio and longer time since last crop residue was returned to the soil increased PNM and NO3-N contents. In contrast, increased residue amount returned to the soil, total rate of N fertilization or last applied N, and shorter time since last residue was returned to the soil increased MBN. As a result, crop growth and yield can be significantly affected. Because of different levels of labile and available pools of N, N fertilization rate needs to be varied between crop rotations, phases of the rotation, and tillage practices for optimal crop production.
                                Technical Abstract: Soil and crop management practices may alter quality, quantity, timing, and placement of crop residues in the soil that influence turnover rate of organic matter, microbial activities, and N mineralization. We examined the effects of two tillage practices [conventional till (CT) and no-till (NT)], five crop rotations [continuous spring wheat (Triticum aestivum L.) (CW), spring wheat-fallow (W-F), spring wheat-lentil (Lens culinaris Medic.) (W-L), spring wheat-spring wheat-fallow (W-W-F), and spring wheat-pea (Pisum sativum L.)-fallow (W-P-F)], and a Conservation Reserve Program (CRP) planting on soil labile C and N pools at 0- to 5- and 5- to 20-cm depths in March 2004 in drylands of the northern Great Plains. Each phase of the crop rotation (W-F, F-W, W-L, L-W, W-W-F, W-F-W, F-W-W, W-P-F, P-F-W, and F-W-P) was included in every year. The C and N pools included microbial biomass C and N (MBC and MBN), potential C and N mineralization (PCM and PNM), and NH4-N and NO3-N contents. A field experiment was conducted in the soil mapped as a mixture of Scobey clay loam (fine-loamy, mixed, Aridic Argiborolls) and Kevin clay loam (fine, montmorillonitic, Aridic Argiborolls) from 1998 to 2003 in Havre, MT. The MBC, PCM, and NH4-N contents were not influenced by treatments and averaged 1548, 210, and 5 kg ha-1 at 0- to 20-cm, respectively. The MBN at 0- to 5-cm was greater in P-F-W (42 kg ha-1) than in CRP, F-W, and W-W-F in CT, but was greater in F-W-W (35 kg ha-1) than in W-F, F-W, L-W, and W-F-W in NT. At 5- to 20-cm, MBN was greater in W-F-W (137 kg ha-1) than in other rotations, except in CW and P-F-W, in CT. Similarly, PNM at 0- to 5-cm was greater in W-P-F (13 kg ha-1) than in other rotations, except in W-L, in CT. The NO3-N content at 0- to 5-cm was greater in W-W-F (31 kg ha-1) than in other rotations, except in W-F and W-P-F, in NT. At 5- to 20-cm, NO3-N was greater in W-P-F (64 kg ha-1) than in other rotations, except in W-L, in CT, but was greater in W-W-F (44 kg ha-1) than in CRP, CW, and P-F-W in NT. Differences in soil and crop management practices and environmental conditions among rotations and phases of rotations altered N mineralization and availability in CT and NT in dryland soils, which could influence crop yields.


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