Due to the fact that the mechanism of acquisition of phosphorus (P) by roots, is mainly by interception, sufficient P uptake is only ensured by maximal root development ("exploitation"). Pot and field trials were conducted to determine the percentage exploitation of the soil volume by roots. The influence of P on root growth of Zea Mays was also studied. Previously, roots were described in terms of root density (cm cm-2, cm cm-3, gram cm-2 and gram cm-3). In this study roots are described in terms of exploitation which combines length, mass and the rhizosphere. The Gompertz function was used to model exploitation by roots as influenced by P application. P along with nitrogen and potassium, had a highly significant (P < 0.001) effect on root growth in the pot experiments. The root systems' function changed after 14 days from nutrient acquisition to shoot supportive. P had no significant effect on root growth in the field trial. Growth was governed by soil moisture, as dryer positions exhibited higher growth. The high P plot had much less root growth in the subsoil than the low P plot. Gompertz functions revealed subtle differences between different treatments. During the first two weeks (when most P uptake occur) roots exploited at the most 1 % of the top soil volume. This implies that any soil analysis (Bray-1 value), should be divided by ≈ 100 to render the "exploitable" P. When considering the total P uptake of a maize crop (5 kg P ton-I), this means that the crop acquires only ≈ 6% of its P from the "plant available" pool (that is represented by the Bray-1 value). This suggests that roots are indeed able to extract the P from "plant unavailable" pools. Therefore, the term "plant available" is misleading and not descriptive concerning P uptake, and its use should be discontinued. / Dissertation (MSc (Soil Science))--University of Pretoria, 2006. / Plant Production and Soil Science / unrestricted

L'adozione di (agro)ecosistemi sostenibili viene indicata come una efficace strategia in grado sequestrare carbonio (C) nel suolo, mitigando così il cambiamento climatico e migliorando la fertilità. Sebbene il potenziale di sequestro del C della non-lavorazione (NT) sia stato generalmente sovrastimato, esso risulta essere di 0,26 Mg ha-1 anno-1 superiore rispetto al regime arativo. Inoltre, il 76,6% di questo quota è localizzato in frazioni considerabili come relativamente stabili. Il NT aumenta lo sviluppo radicale delle colture erbacee (es. mais, soia, frumento) negli stati superficiali del suolo (0-5 cm). Le correlazioni tra i parametri di densità radicale e le proprietà fisiche del suolo mostrano come lo sviluppo radicale sia un fondamentale indicatore di qualità del suolo in NT. I residui delle cover crops influenzano le emissioni di protossido d’azoto (N2O) in NT: i residui di segale favoriscono l'immobilizzazione dell’azoto (N), aumentandone così l'efficienza d’utilizzo e diminuendo le emissioni, mentre i residui di veccia vellutata aumentano l’N2O come conseguenza della mineralizzazione dell’N. Le emissioni di N2O e la produttività dei prati stabili possono essere positivamente correlate, perché meccanismi diversi rispetto alla regolazione indotta dalla disponibilità di N possono controllare l'N2O: il C potrebbe essere un principale fattore di regolazione per nitrificazione e denitrificazione. / Adoption of sustainable (agro)ecosystems has been widely suggested to increase soil organic carbon (C) sequestration, to mitigate climate change and enhance soil fertility. Although its carbon sequestration potential has been generally overestimated, no-till (NT) results in an extra C sequestration of 0.26 ± 0.18 Mg ha-1 yr-1 as compared to conventional tillage and 76.6% of this extra C is located in C pools which could be considered relatively stable. NT increases root development of field crops (i.e. maize, soybean, winter wheat) in the top soil (0-5 cm), while does not in the deeper soil (5-60 cm). Positive correlations between root density and soil physical parameters shows how roots are main drivers of soil physical properties under NT. Cover crop residues may affect nitrous oxide (N2O) emissions under NT: rye residues enhances soil-nitrogen (N) immobilization, thus increasing N use efficiency and decreasing N2O, while hairy vetch residues as cover crop under NT increases N2O as a consequence of soil-N mineralization. N2O emissions and shoot productivity may be positive correlated in grasslands, because other mechanisms than plant-induced regulation of soil N pool may control N2O: C could be a major factor regulating nitrification and denitrification processes.