The effect of long-term no-till crop rotation practices on the soil organic matter functional pools

Smith, Jacques De Villiers (2014-04)

Thesis (MScAgric)--Stellenbosch University, 2014.

Thesis

ENGLISH ABSTRACT: Total soil organic matter (SOM) and its different functional pools (fractions) are important attributes of the physical, chemical and biological quality of the soil and are seen as key factors in the evaluation of the sustainability of management practices. Until now, limited information was available regarding soil C accumulation and stabilization under conservation tillage managed soils in the Western Cape grain production regions of South Africa. Long-term field experiments investigating different crop and crop/pasture rotation systems under no-tillage were initiated in 2002 at the Tygerhoek Research Farm of the Western Cape Department of Agriculture, near Riviersonderend, Overberg, Western Cape, South Africa. The study site enabled us to compare the following five dryland cropping systems; permanent Lucerne (100% pasture), Medic-Medic-Wheat (MMW) (67% pasture; 33% crop), Medic-Medic-Wheat-Wheat (MMWW) (50% pasture, 50% crop) and two 100% cropping systems (continuous cropping) in different phase [Wheat-Barley-Canola-Wheat-Barley-Lupin (WBCWBL4 & WBCWBL1)]. The numbers “1” and “4” in rotation code refers to the first and fourth crop planted in the cropping system, respectively. The underlined crop in rotation code represents the crop that was on the field at time of sampling. Natural vegetated soil (non-cultivated area) acted as a reference for this study. In 2012, soil samples were taken at four depth increments; 0-5, 5-10, 10-20, 20-30 cm. The objectives of the study were to investigate the effect of long-term crop/pasture rotation systems on: i) the total soil organic carbon (SOC) storage under different cropping systems, ii) the SOC and N content in different functional pools (fractions); free particulate organic matter (fPOM) fraction (labile fraction), occluded particulate organic matter (oPOM) fraction (moderately stabile intra-aggregate C) and mineral-associated fraction (stabile fraction), (iii) the main C stabilizing mechanisms operative in these soils and (iv) the relationship between the extent of C sequestration and crop yields. After 11 years, the medic-wheat rotations had the highest total SOC contents (15.2-18.6 g kgˉ¹ in 0-30 cm depth, P ≤ 0.05), compared to the continuous cropping (13.3-14.1 g kgˉ¹ in 0-30 cm depth), permanent lucerne pasture (15 g kgˉ¹) or natural vegetated soil (13.2 g kgˉ¹). Higher belowground C inputs through roots and the lower extent of disturbance in the 0-10 cm depth are the main reasons for higher total C content in the wheat-medic systems compared to the other systems. The contribution of the fPOM fraction (labile C) to total C content in the cultivated treatments (6-9%) was lower than the natural vegetated soil (13%) in the 5-10 cm depth. The fPOM fraction is the most sensitive soil organic C and N pool to detect changes due to management practices, which include quantity and quality of OM inputs, extent of physical disturbance, and fertilization. The medic-wheat rotations had the highest C (1.37-1.74 g kgˉ¹ in 5-10 cm depth) and N (0.107-0.110 g kgˉ¹ in 5-10 cm depth) contents in the fPOM fraction of the cultivated treatments. Compared to the natural vegetated soil, the cultivated treatments had a lower C content in the oPOM fraction (moderately stabile fraction) and concomitantly a lower aggregate stability. On average, the oPOM fraction only contributed 0.4-2.4% to total C content at all sites. A significant positive correlation (R²= 0.77) was found between C occluded in aggregates (oPOM fraction) and aggregate stability with the highest aggregate stability found in the medic-wheat rotations of the cultivated treatments. The major part (85-93%) of the SOC was associated with the mineral fraction (stabile fraction) in the natural vegetated and agricultural soils. The MMWW treatment contained the highest C content (18.7 g kgˉ¹, 5-10 cm depth) in the mineral-associated fraction and the two continuous cropping systems the lowest (14.2-14.7 g kgˉ¹, 5-10 cm depth) of the cultivated treatments. A significant positive correlation was found between mineral-associated SOC fraction and clay (R² = 0.74) and Fe-oxide (R² = 0.57) content. This helps explain the large mineral SOC fraction found in these soils and is the dominant SOM stabilization mechanism operative in these shale-derived soils. The mineral-associated organic matter is probably predominantly sorbed to the clay minerals (illite, kaolinite and sesquioxides) via ligand exchange resulting in very strong organo-mineral associations. Physical protection via occlusion in aggregates is not a dominant C stabilizing mechanism in these soils. The C:N ratios of the fractions decreased in the order fPOM > oPOM > mineral with a C:N ratio below 10 in the mineral fraction indicative of humified organic matter. The MMW and MMWW treatments produced higher wheat yields in 2012 with a significant positive correlation found between total soil C and N, and yields obtained. In a higher quality soil, higher agronomic production is expected. Findings in this study enabled us to conclude that due to effect of cropping system and soil properties, the MMWW treatment had the highest total SOC content, which included highest labile C and N content and highest.

AFRIKAANSE OPSOMMING: Totale grond organiese materiaal (GOM) en die verskillende poele (fraksies) is belangrike eienskappe van die fisiese, chemiese en biologiese kwaliteit van grond en word gesien as belangrike faktore in die evaluering van die volhoubaarheid van bestuurspraktyke. Tot nou was beperkte inligting egter beskikbaar rakende grond koolstof akkumulasie en stabilisering in gronde onderhewig aan bewaringslandbou in die graanproduserende streke van die Wes-Kaap. In 2002 is langtermyn plaasskaal wisselbou proewe op die Tygerhoek Navorsingsplaas van die Wes-Kaapse Departement van Landbou, naby Riviersonderend in die Overberg (Suid-Afrika) geloots. Die studiegebied het dit moontlik gemaak om die volgende vyf droëland gewasverbouing stelsels (behandelings) te vergelyk: permanente Lusern (100% weiding), Medic-Medic-Koring (MMK) (67% weiding, 33% gewas); Medic-Medic-Koring-Koring (MMKK) (50% weiding, 50% gewas) en Koring-Gars-Kanola-Koring-Gars-Lupien (KGKKGL4 & KGKKGL1) (100% gewas). Die nommers “1” en “4” in rotasiekode verwys na die eerste en vierde gewas geplant in die rotasie stelses, onderskeidelik. Die onderstreepte gewas in rotasiekode verteenwoordig die gewas in die veld toe monsterneming plaasgevind het. Grond onderhewig aan natuurlike plantegroei (onbewerkte gronde) het gedien as verwysing vir hierdie studie (sesde behandeling). In 2012 was grondmonsters geneem op vier verskillende dieptes; 0-5, 5-10, 10-20 en 20-30 cm. Die doelwitte van die studie was om ondersoek in te stel oor die effek van langtermyn gewas/weiding wisselboustelsels op: i) die storing van totale grond organiese koolstof (GOK) inhoud onder verskillende verbouingstelsels, (ii) die GOK en stikstof inhoud in die verskillende funksionele poele (fraksies); vrye fraksie (VF), ingeslote (intra-aggregate) fraksie (IF) en mineraalgebonde fraksie (MF), (iii) die hoof koolstof stabiliserings meganismes in werking in hierdie gronde (iv) die verhouding tussen die omvang van koolstof sekwestrasie en opbrengste. Na 11 jaar het die medic-koring rotasies die hoogste totale koolstof inhoud gehad (15.2-18.6 g kgˉ¹ in 0-30 cm diepte, P ≤ 0.05), in vergelyking met volgehoue verbouing met kontantgewasse (13.3-14.1 g kgˉ¹ in 0-30 cm diepte), permanente weiding (15 g kgˉ¹) en natuurlike plantegroei (13.2 g kgˉ¹). Hoër ondergrondse koolstof insette deur wortels en die mindere mate van versteuring in die 0-10 cm diepte is die vernaamste redes vir die hoër totale koolstof inhoud in die gewas weiding stelsels. Die bydrae van die vry fraksie (labiele koolstof) tot totale koolstof inhoud in die bewerkte behandelings (6-9%) was laer as die van natuurlike plantegroei (13%) in die 5-10 cm diepte. Hierdie fraksie is die sensitiefste poel van organiese koolstof en stikstof om veranderinge weens effek van bestuurspraktyke, wat die kwantiteit en kwaliteit van OM insette, mate van versteuring en bemesting insluit, op te spoor. Die medic-koring rotasies het die hoogste koolstof (1.37-1.74 g kgˉ¹ in die 5-10 cm diepte) en stikstof (0.107-0.110 g kgˉ¹ in die 5-10 cm diepte) inhoud in die vrye fraksie gehad van die bewerkte behandelings. In vergelyking met die grond onder natuurlike plantegroei, het die bewerkte behandelings ‘n laer koolstof inhoud in die ingeslote fraksie (gematigde stabiele fraksie) gehad weens ‘n laer aggregaat stabiliteit. Die ingeslote fraksie het gemiddeld net 0.4-2.4% bygedra tot die totale koolstof inhoud in al die behandelings. ‘n Beduidende positiewe korrelasie (R²= 0.77) was gevind tussen intra-aggregate koolstof (ingeslote fraksie) en aggregaat stabiliteit met die hoogste aggregaat stabiliteit in die medic-koring rotasies van die bewerkte behandelings. Die grootste deel (85-93 %) van die totale GOK inhoud hou verband met die mineraal fraksie (stabiele fraksie) in beide die natuurlike plantegroei en landbougrond. Die MMKK behandeling (18.7 g kgˉ¹, 5-10 cm diep) het die hoogste koolstof inhoud in die minerale fraksie gehad met die twee 100 % gewas wisselboustelsels (14.2-14.7 g kgˉ¹, 5-10 cm diepte) die laagste van die bewerkte behandelings. ‘n Beduidende korrelasie tussen minerale koolstof (mineraal fraksie) en klei (R² = 0.74) en Fe-oksied (R² = 0.57) inhoud is ook gevind wat die groot bydra van die mineraal fraksie tot totale koolstof inhoud help verduidelik. Dit is ook die dominante GOK stabiliserings meganisme in werking in hierdie skalie-afkomstige gronde. Dit blyk dat die mineraal geassosieerde OM oorheersend aan die klei minerale (kaoliniet, illiet and seskwioksiedes) adsorbeer d.m.v ligand-uitruiling wat baie sterk organiese-mineraal komplekse vorm. Fisiese beskerming d.m.v. insluiting binne aggregate is nie ‘n dominante koolstof stabiliserings meganisme in hierdie gronde nie. Die C:N verhouding van die fraksies het afgeneem in die volgorde VF> IF> MF met 'n C:N verhouding onder 10 in die mineraal fraksie wat ‘n aanduiding is van gehumufiseerde OM. Die MMK en MMKK sisteme het hoër koring opbrengste in 2012 tot gevolg gehad en beduidende positiewe korrelasies was gevind tussen totale koolstof en stikstof en opbrengste. In ‘n hoër kwaliteit grond word hoër opbrengste verwag. Bevindinge in hierdie studie het gelei tot die gevolgtrekking dat a.g.v. die rotasie sisteem en grond eienskappe, het die MMKK behandeling die hoogste totale koolstof inhoud gehad. Dit sluit die hoogste labiele koolstof en stikstof inhoud asook die hoogste stabiele koolstof inhoud in.

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